Replaced ENTIRE brake system - brakes still not working properly!
 

I've had a problem with the brakes on my 78 T/A since I bought it a little over a year ago. I initially had a mechanic replace the M/C, prop valve, and parts of the brake lines, but that didn't seem to help much. I then replaced the drums/shoes/cylinders and calipers/pads - no significant change. After chasing the problem around, I ended up replacing the booster and M/C (this is now the second M/C, and I also bled it). The problem persisted - the brakes don't stop the car from a roll until late in the pedal's travel. I then decided to replace all of the lines (including the rubber lines) and the proportioning valve (again). Now every single part in the brake system is new. I pressure bled today, having just finished installing the lines, and everything seemed to bleed fine - some bubbles, then solid fluid, from all four corners.

The problem is the same - although the pedal feels slightly different. The pedal is stiff when the engine is not running, and when it is running, it's feather light, there's almost no resistance. The car won't stop from a roll until the pedal is floored.

Anyone have any idea what the problem might be? Absolutely everything is new. Is there any kind of adjustment on the pedal? I know there's two holes - the booster rod is on the lower hole. The upper hole has a screw and a nut through it. How much fluid do I need to bleed after replacing all of the lines - maybe I didn't bleed enough?

Any help would be greatly appreciated - I'm at a total loss here. Thanks!

Since it seems you have been screwing around with this for a long time now,I'm sure you have tried this,but I have to ask since I didn't see you mention it.
Have you tried to adjust the rear brakes up tighter? If the rear drums are too far out of adjustment,it will have the same symptoms.
Just checking.;)

Hey thanks for the reply-

I have done this a few times as I've chased this problem around - but I haven't done it recently, so I'll give it a shot.

How much would you tighten them? I've read all different ways to know when they're set. I think the method I used last was to tighten them until they drag, then back it off a little.

Did you check the clearances between the master and booster? That could account for excessive pedal travel. Honestly though, my best guess, and it's only a guess, there is still an issue with bleeding the brakes.

What clearances? How do I check that?

Do you know how much I should bleed the system before it's safe to assume there's no air anywhere? I agree that it sounds like air is somewhere - I just don't know where.. or how :confused::confused:

Pedal travel comes to mind.I believe there is an adjustment that you can make on the brake pedal rod.If for some reason the rod length has been messed with prior to your ownership this could throw everything out of whack!I'd first check the rod length to another rod from another bird.At least it would tell you if your rod length is OK.Goodluck!

Well, I have been having the opposite problem, too soon of a pedal that will put your face in the dash but I do know that on my 63 GP, there is an adjusting screw on the end of the booster rod that sticks out and goes into the MC. It is a smal ball type end adjusting screw that takes a 1/4" wrench and the other (female) section takes a 5/16". I'm not sure if it is adjustable like mine or it may take a ball or something like that in between the MC and rod? Sounds like this is where your problem is?

Thanks for the replies-

This is what throws me off: The pedal is stiff all the way from the beginning of its travel when the engine isn't running. I don't know much about the inner workings of the booster, so would the booster alone cause the pedal to be stiff when the engine isn't running? I've been assuming that it's stiff because the booster is making contact with the M/C.

If that is the case - the booster is making contact with the M/C from the start of the pedal's travel - then if I adjusted it, it would constantly be depressing the M/C piston - is this what should be happening, or should the booster come completely off of the M/C when there's no pressure on the pedal?

There should be no contact between the booster rod and the master cylinder, only the slightest clearance to allow for heat expansion. If the booster rod is depressing the master cylinder just by being bolted together, the system may not bleed properly since the master cylinder is never fully at rest. Try loosening the bolts for the master, pull it forward from the booster just slightly, then bleed the brakes again using a vacuum pump at each wheel.

Sounds like it is definitely a rod problem. However it seems you still mdon't have them bled all the way too. Let that TA gravity bleed for a while. I imagine the second gens are similiar in fluid capacity, about two quarts. Bleed all the way through til you get nice clean fluid out the rear. Good luck

OK thanks - I'll give that a try. I don't think the booster is depressing the M/C just by being bolted together, but I'm not 100% sure. If I do determine that the booster is depressing the M/C, what should I do to fix that?

I'll give gravity bleeding a shot if the above doesn't work. I've heard mixed opinions on gravity bleeding - but at this point I'll try anything. Is there an advantage to gravity bleeding over pressure bleeding? One of the problems is it's hard for me to see when I've bled the entire system, since it's all new fluid.

Gravity bleeding is a pain, especially now, when you want to be driving it not chasing down bugs. I've just always jacked the car up level and cracked all four bleeders and let her go. Fill up master and wait an hour, come back and pour more fluid in. It takes all day but then your manual bleeding takes a half hour. Then do it again after a couple of days, the manual bleeding that is. Good luck, hope ya get squared away.

I am having the exact same problem with my '78 TA except I have a totally new system in there: '99 TA Rear brakes up front on 1LE rotors and '86 GN front disc brakes on the back...no E-brake. I have a Detroit Speed master/booster combo and no matter how much I bleed it as soon as I fired it up there is basically no pedal. EXACTLY like yours...hard when off, nothing when running.

GREAT to hear from someone else having the same problem...at least I am not crazy. I will try and adjust the push rod length and see what I get.

Dave

I feel the same way! I'm glad I'm not the only one out there. It's such a frustrating problem. My old mech couldn't even figure it out - he told me my cam was too big, so my vacuum was too low - but wouldn't that make the pedal hard??

Anyway, if you do end up solving it, PLEASE let me know, and I'll be sure to do the same.

Do your brakes work a little? With the car running, when I floor the pedal, I can stop it from a roll.

EDIT: I was just reading another thread, and someone mentioned that he's seen a lot of brand new M/Cs that are bad right out of the box - I've actually heard that a few times. Maybe it's possible that's my problem? Two bad M/Cs in a row? Are my symptoms indicative of a bad M/C?

-Jimmy

You still have air in the master cylinder. Either take it off and bench bleed it properly, or find a hill steep enough that the master cylinder is level when the car is parked nose-down and do short-stroke pumps on it for a while. The bubbles will come up from the holes in the floor of the master cylinder reservoir.

Hey Jim,

I hear ya! I have been working with Detroit Speed and they have been awesome...they went back to their manufacturer and after a day or two they sent me a new booster.

My car stops without the booster hooked up but as soon as I hook the booster up it takes virtually no pressure and the pedal goes to the floor. Too afraid to try and stop it with the booster hooked up.

I will try, once again to bench bleed the master. I am going to install speed bleeders as well to see if I can luck out there as well.

I'll let you know,
Dave

Well...I tried to bleed the brakes again. I did get a few big bubbles from the front passenger side but no change in vacuum assist issue. I tried plugging the master and bench bleeding and no change again...at this point it has turned into a full blown quest!!!

I am not sure at this point if the booster is really bad. I still think there is air hidden in the system somewhere. I will try and completely remove the master cylinder to try and bench bleed it while being level.

I used a mitivac to bleed it and I am still having no luck...anybody got any ideas for us???

Thanks,
Dave

i had a similar problem on a vehicle. turned out the master cylinder was wrong for the car. the dept of the rear of the master cylinder where the brake booster rod goes was deeper than the original. drove me crazy for a week. i thought it was trapped air. Remove the master cylinder and measure the dept of the rear hole. Now measure the extending rod end from the booster where the master cylinder will bolt on. it should be real close. check your manuel for the exact measurment needed. most booster rods have a adjustment on the end to lengthen or shorten them. mine was off darn near 1/2"

I am going to try something similar this evening...Detroit Speed and Engineering sent me a brand new booster so i would have to take off the master anyway...

I figure I will measure the pushrod while I have the opportunity.

Keep you all posted!
Dave

well it is more or less a process of elimination. if you have air in the lines you will see boubbles in the fluid when you bleed them. if you check the dept of the rear master cylinder against the lengh of the rod into that hole where it makes contact then that is eliminated. do you have the correct kind of hose from the intake to booster. a lot of people use gas line hose, instead of the heavier booster hose. That can collapse when the brakes are applied. also check the vacume one way valve on that line at the booster and make sure it is working properly.

There are two possible pedal considerations: 1.) the positioning of the swivel connector (that takes the pin), and the proper hole in the pedal arm, and 2.) the length of the pushrod that sits between the booster and the master cylinder...there are usually two pre-fit sizes and then an adjustable unit when you buy a new booster. I'm thinking it might be that you have too short of a pushrod in there or your prop valve is frozen or misadjusted.

I had a horrible time when I replaced my MC and booster last year. Hopefully something in my story will help.

After the swap, with everything else unchanged, my pedal would get stiff and travel would become very short after a qhick drive. Ended up the front brakes also started to drag so badly that I couldn't even turn the tire by and after jacking it up. After it cooled down everything was fine. Pedal travel was adjusted all the way out but I couldn't give it enough not to run out of travel. I thought it was air in the MC, brake lines too close to headers, etc.

Read another post that ponted to some shoddy clearances between the MC and booster, specifically the rod. Ended up the MC was being engaging on me. Try this. Loosen the two MC to booster bolts. The MC should just sit there. In my case the MC wanted to pop off the face slightly. Nothing that made me think there was a problem. I threw some washers on the two mouting posts and I never had a problem again. Ended up the piston was being depressed slightly and after the brake fluid heated up I was getting enough engagement to drag the front brakes.

My problem has to do with a DSE booster/master cylinder combo...the pushrod comes with already installed in the assembly. I am going to try something tonight with regards to the brake bleeding/pushrod length and see if it works.

Thanks,
Dave

Hey sorry I haven't checked on this thread in a while - I'm in my last two weeks of college, so I haven't had much pontiac-time.

I missed too much to respond individually, but I carefully read everyone's post. As many of you mentioned, there's just not that much to the system. I'm going to take the M/C off, bench bleed it again, and measure the rod length while it's off. That should eliminate two things. If that doesn't pan out, then I'm leaning towards two bad masters in a row. I've read a few people's stories of receiving two bad masters in a row, and it really throws you off, because after replacing it twice, you just assume that's not the problem.

Someone mentioned that the vac line could be wrong and collapsing or the valve could be malfunctioning, wouldn't that cause a lack of vacuum to the booster, thus making the pedal hard? This is an issue that I keep hearing in various forms - particularly when talking about my cam - that low vacuum can cause issues like this. Can someone with knowledge of these power boosters clear this up for me? I was under the impression that low/no vac would just cause a stiff, but still working, brake pedal, since the power assist wouldn't be operating. If I'm mistaken, then that introduces a few other possible causes for my problems.

Dave: I'm glad you're on your way to solving this! Definitely keep us updated!

i just mentioned the valve and hose as a area for you to inspect to be sure they are functioning properly. i don't believe that is your problem. If you have trouble finding booster hose, just ask for air brake hose like the trucks use. I am still leaning toward master cylinder hole debt, and booster rod length. A lot of the books now only list one master cylinder for power/none power brakes for the old cars. In the old days there were two different master cylinders listed on most models. These kids behind the counter now are too young to know that. They just go by what the computer says. also have you checked for any tight bends in the lines from the master cylinder to the prop. valve? You stated the brakes worked, just late in the pedal trave. and like gtogreg said pin location on the two pre drilled holes on the pedal. providing your car has that.

OK no problem - I wasn't sure if that was just additional advice, or if I was misunderstanding how the booster functions. The lines are all new from inline tube, but I did have to play with the M/C -> prop valve lines to get them to fit. I was careful to only make big bends, but I'll double check.

I did check the pin location, and it's in the lower hole - which I *think* is the correct one, please let me know if it isn't. The upper hole has a bolt and a nut through it, which appears to serve no purpose other than to fill the hole (it's tough to get myself in to a position where I can easily see the holes).

Hey Guys,

I think I am on to something...I bench bled the master...again. No change...

Replaced the booster and and there is definitely a change for the better. I have to rebleed the system again and check it but I will keep you posted for tonight. It just got too late last night and I had to go to bed...I know...big baby.

Stay tuned...
Dave

Hey Dave,

Did you ever make any progress with this? I haven't had any time to look at mine, but I'll finally have time to work on it a lot next week.

Jim

UPDATE: I took the M/C off to bench bleed again. While I had it off, I took a look at the booster rod length - it looks fine. I also pressed the pedal all the way down and measured how far the rod comes out of the booster. That length is also roughly the same as how far I can manually push the piston in the M/C.

I bench bled the master, and got some bubbles. I kept getting small bubbles out of the front reservoir if I let it sit for a few minutes, then bled again. I decided to bolt it on, but leave the hoses attached to the two ports and running back in to the reservoirs. I then pumped the pedal, and the rear reservoir was fine, no bubbles whatsoever, but very small bubbles kept coming out of the front port. When I pressed the brake pedal all the way down and held it there, little bubbles would continue coming out of the front port for a few seconds. I kept pumping to try to bleed that air out, but I couldn't get to the point where there were no bubbles.

I decided to hook the lines back up, bleed the system, and see if there was a change. While I was bleeding the rears, I also adjusted the shoes so that they make a good amount of contact with the drums (I'd have to tap the drum with a mallet to get it off). Either the bleeding or the rear adjustment caused the pedal to be a LOT more stiff when the engine isn't running. It feels exactly as it should - so tough that I feel as though I might break the pedal if I press it all the way down - but it still goes down, just with a lot of force.

Unfortunately, with the engine running, the pedal is still feather light, and I'm still having the exact same problem. The car doesn't stop until the brake pedal is almost completely on the floor.

What do you guys think? Bad M/C? I'm pretty sure I covered all of my bases. I pressure bled the brakes for a l-o-n-g time, there's no air in the lines.

Replace the master. Is it one of the part store specials? If so try to get one from a reputable manufacturer.

It's a new unit that I purchased at Advance Auto, I forget who actually manufactured it. I don't know if that makes it a part store special or not. When I bought it, I wasn't aware that there was much difference if I was just replacing the original master.

Can you recommend any manufacturers?

I've still been trying to solve this problem, but haven't had much luck.

While looking in to something else, I came across a diagram of how a front disc/rear drum brake system should be plumbed, and it showed a residual valve for the rear brakes, and said it's very important. I just replaced all of my brake lines, and didn't see any residual valve. Is that built in to the combination valve, or is it something I need? Does anyone know?

Change the booster...

I've already replaced it. Could the booster cause these symptoms? I'd replace it again, but I thought a bad booster would cause a hard pedal?

also check to see what bore size master cylinder you have, the small bore 1'' or smaller are for manual brakes, yours should be the larger more than 1'' for power brakes

Yep, I thought (was hoping) that was the problem, but I measured the bore size of my previous master, and it was 1 1/8", which I believe is the correct size. I then called Cardone for the bore size of the master that I currently have installed, and they said that it's 1 1/8".

I'm going to start isolating parts of the system today, starting with plugging the master, and seeing if the pedal stiffens up, that way I'll know if it's the booster/master, or something else, and I'll move around the system in that fashion, hopefully until I figure out the problem. If anyone has any suggestions, I'd be happy to hear them - I'm getting desperate! It's getting nice out, and I need my T/A to stop as well as it starts!

maybe the proportioning valve is out of adjustment, most likely its going to be something simple. The other thing I would do is find someone who has the exact same car as you , take a look how everything is setup on theirs then verify it with yours.

Yeah, I'll take a shot, since there's a couple of grey areas still.

On the rear brakes, adjust them all the way to the drum, with it up on jack stands, and try the pedal with the car running, see if it's the same. Drums, adjusted, drag. If the shoes aren't centered properly on the backing plate, or there's something else out of whack back there, it can have this type of an effect. Take a pick of both sides with the drums off and post them.

You verified the booster push rod and MC plunger distance? What is it? It's supposed to be something like < 3/16s, not something you can eye-ball. You should be using the ears of the MC mounting area as a reference, and that suface on the booster. If there's a question, use a small piece of wax or something on the end of the rod, tighen the MC up, take it off, and see how far it's compacted.

MC bleeding. NO bubbles should come out, and you keep pumping that b*tch until there are NO bubbles. It takes a while, but you do get there. Don't even bother putting the thing on the car if you still see bubbles. You can tap the side with a wrech lightly in between pumps, it helps a little. You don't have to depress the punger all the way, and in fact, think you're not supposed to bottom it out.

MC sizing, yup, you'll probably have a 1 1/8 MC. Those are the type with shallow rod pucks. The 1" manual MCs ONLY come in deep rod puck/hole, which is what some people want, and they still don't make any. Some people have used spacers to use a 1" on a booster. Regardless, you have a 1 1/8, and if it's a 78 T/A, it's the typical GM large/small reservoir type MC. Take a pic, post it.

MC line placement. You say you replaced the prop valve? Which line do the rear brakes plumb to on the master? Usually, the front port on the MC goes to the rear brakes, and the rear ports go to the front brakes. If there's a question if this is correct or not, check the factory service manual. You say you had to 'bend' the MC to prop lines as shipped from Inline to make them work? Sounds like you might have them mixed up. Take a pic and post it.

Prop valve- Does the brake light in the dash come on when you hit the brakes all the way? If so, your prop valve brake bias rod may be all the way to one side, which generally, only effects your rear brakes. Do a search on pro-touring for 'brake bias switch'.

System bleeding- A quick note on this, gravity is ok, but usually on a fresh system, it's hard to get started. In general, however you bleed the system, always start from the furthest wheel from the MC, then the next furthest, etc. Never let the MC go dry, since you'll be re-entering air into the system, and that bubble will have to travel the entire distance to remove.

Booster replacement - You can unbolt and move the MC out of the way enough to where you don't have to take the lines off and re-bleed.


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THANK YOU for your informative response!

I got very excited, because I thought that the lines from my M/C to the combo valve might be wrong, because I have seen many cars with the lines crossed, but I verified in my manual that the rear (closest to the firewall) port on the M/C is for the rear brakes, and the front (closest to the front of the car) port is for the front brakes. The lines from the M/C are two sizes - the thinner one goes from the front port to the left (facing the firewall) port on the combo valve (the one for the front brakes), the thicker one goes from the rear port to the right port on the combo valve (for the rear brakes). Does that sound correct? I can post a picture if that helps.

I have adjusted the rear drums until they drag, but I will take pictures some time today.

I have not verified the booster pushrod distance. Is there a way I can measure this, or should I use wax or something as you described?

I haven't actually been able to get the warning switch connected. It seems as though the rod is too high for the connector to fit, but I hadn't played with it very much. The valve is new from inline tubes.

I've bled the brakes every possible way - traditional (using the pedal), vacuum, gravity, and most recently, pressure.


I do recall there being a change in the feel of the pedal once I replaced the booster. I hadn't thought about it, because I was under the impression that a bad booster would cause a stiff pedal. However, I've read recently that a bad booster can cause a low pedal, too. Is that true? If so, is there any way that I can test the booster?

Thanks again for your reply - I really appreciate the help. I'm getting very frustrated with this problem.

Yeah, I honestly can't remember the orientation of the lines/ports on the 2nd gen F-bodies, I ditched mine years ago for an adjustable. If you looked at the factory service manual, and it's clear which goes where, then obviously you're good with that.

On the rear adjustment, crank those things out there until you feel resistence in the adjuster, like with it applying brakes. This is just for troubleshooting to see if the pedal changes. If it does make a diff, you know where the problem may lay.

Pushrod measurement, well, may sound cheesy, but I used a paperclip bent from the flange to the cup, then measured the paperclip. Worked for me. If there's an issue with the pedal as you're talking about, it's going to be obvious. I kinda don't think that's it, because you say there's no 'loose' or 'free' travel of the pedal. You can take the MC cover off, and as you depress the pedal, you should see fluid movement, can try that as a general indicator.

Power/pressure bleed is the best, period. If you've got a rig for it, that's the way to go. Same thing, start at the furthest wheel, and make sure the tank is full.

Warning switch, that will tell you if the prop is boogered, in most cases. If it's a bias rod problem, the light will come on.

On the pedal, in the car, the upper hole is for manual brakes, the lower for power, or that's how I remember it. Wouldn't hurt to try a different one I guess. For a manual setup, using the power hole will make the pedal hard.

Hm, a bad booster causing a low pedal, not sure on that one. I know when there's no/low assist, due to a bad booster, the pedal is hard. Someone once told me that when you have your foot lightly on the pedal, and you start the car, the pedal should slowly go down, which indicates a good booster, but don't know how true that is. Do you have the original factory booster still? Might be able to try it. If you bought the booster from DSE, they should be troubleshooting it with you. Call them.

Thanks, I'm going to play with the rear shoe adjustment in a few minutes.

I like your pushrod measurement method lol, I'm going to give that a shot, too.

Yeah, after bleeding for hours and hours, the $60 bucks or whatever it cost for the pressure bleeder just seemed worth it.

I'm using the lower pedal hole - so I don't think that's the issue, unless there's something else out of alignment down there.

I wasn't sure about the booster either. I didn't get it from DSE, it's from AutoZone, I don't recall the brand. I could figure it out and give them a ring though, that'd at least rule that out.

The warning switch sounds like it'd be a big help - but I don't know how to get the connector to fit. I'm pretty sure I broke the connector trying to get it on, because I had it on the pin that comes out of the combo valve, and then I tried to pull the connector back off, and it left a small metal collar on the pin, which I'm pretty sure broke off from inside, so I need to find a new connector. I don't see how the connector could fit, though, it seems like the pin is too high. Is that how it's supposed to be?

I think that I just had a breakthrough though - I had previously thought that my brake pressure gauge didn't come with the right fitting for my calipers, but I found the correct one. I was able to measure the pressure going to the front left caliper, and the highest I measured was 600PSI. I'm supposed to get more like 1200-1500, right? What would cause that low pressure?

Thanks again

Yeah, 600 is way too low, and that's MC and/or booster issues. Or a leak! Well, could be that the rear shoes are traveling too far before hitting the drum too, not enough sweep on the pedal/MC.

Yup, you need a new connector, Help products has them, and since you have to splice anyway, you can add a length. The aftermarket prop valves, some do have a different connector, and may stand off higher, making the original connector not fit well. Honestly, I just said effit, and left mine off.

Hm on the booster, might be the issue. There's some referb shops that do those specifically, the 2nd gens, might want to look into that, could be your problem all together. There was something wierd about the 79-81 boosters, can't recall, but if they list them for 70 1/2 - 81, might be your clue. Check PY, think they're affordable, and they're correct.

MPBrake.com has the push rod depth checker, found that earlier doing a search, never knew they had one! 20 bucks is worth it!

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I hate to jump in so late in the game but i hae seen this 1000 times - it is in the bleeding of the brakes. It takes several times to bleed a freash system. It could take as many as 6 times with a day inbetween for bubbles to settle. The more you bleed the better it will get. All the movement of fluid creates bubbles and the waiting game lets them all collect. We all over think things at times - take a break and go back in a couple of days. hope this helps john

All 60s cars with power BIG brakes like yours come on fast and hard.
Its for you to learn to apply gently and modulate the pressure.
Some of that can be improved with proper adjustment of the leading edge. Check your manual for detailed procedure, and it is a detailed adjustment.
Those big 11" brakes will stop you from any speed if correctly set up but usually only once from high speed and then they need a cool down,
Mine took some getting used to when I first got my 63, drove it 3 years as a daily driver but even though I now drive it very seldom, when I do, the brakes feel normal and familiar.
Wouldnt trade them for disks ever.

Thanks for the replies!

I've spent the last few hours troubleshooting this with a buddy. Here's what we discovered:

• Using an SSBC brake pressure gauge, the front brakes read no pressure until the pedal is almost at the floor (maybe an inch from the floor), then in its final bit of travel, it starts to read some pressure.
• If we stabbed the pedal, we got more pressure - around 500PSI. If we slowly depressed the pedal, we got less pressure, sometimes around 250psi.
• We plugged the M/C ports and tried depressing the pedal, and it was as hard as a rock. We took that to mean that the booster and M/C are fine (no air, no internal leaking) - is that a correct assumption?
• The metering valve pin on the combination valve appeared to be sticking out more than we'd expect (it wasn't flush). We read that when pressure bleeding, it can cause the combo valve to cut off one of the circuits. We couldn't push the pin back in, it just popped right back out. I read somewhere to press it in as the brakes are bled, so we tried that, but as soon as we pressed the pedal, fluid shot out from the metering valve. We didn't do that again :D.
• We bled the front brakes. We kept getting bubbles from the front left, and got solid fluid from the front right. I feel as though there may be more air in the front left, because after letting it sit for a few minutes, we pulled more bubbles. As John said, maybe it's just air?

So what does that sound like to you guys? We feel confident we've isolated the problem to after the M/C. We think it could be an issue with the combination valve, or the rear brakes. We haven't bled the rear brakes yet - it got too late. Could air be the sole cause of these problems? The fact that the pressure gauge reads 0 until the pedal is almost floored makes us think it's not air, but we could definitely be wrong.

Very frustrated update

I put my pressure gauge on my back right wheel, and I get 500PSI when the pedal is near the floor - just like the front brakes. The pressure also decreases around 50-100PSI, slowly, while the pedal is being held. That sounds like a leak to me, but I don't see any leaks. After pumping the pedal, we got around 550PSI. One time, while depressing the pedal extremely slowly, we got no pressure when the pedal was on the floor (combination valve cutting off the rear brakes?). There's probably some air in the rear lines, but could that cause this problem? Even if that accounted for the 300PSI I'm missing from the rear brakes, what's going on with the front? I've bled them quite a bit. I'm not so sure this is air, since the problem existed before I replaced the lines, when I had bled the brakes numerous times.

Does anyone have any clue what could be wrong here? Frustrated doesn't even begin to describe me right now. It's gorgeous outside, and I just want to drive my car. I'm a click away from ordering an SSBC kit and hopefully being done with all of this. I didn't really want to spend the money on that right now, but at this point, if that's what it takes for me to drive my car this summer, then that's what I'll do.

Anyone?

Yes, if you get a hard pedal with the master cyl. plugged, you can eliminate the master cylinder and booster as the problem. So you either have:
(1) A leak
(2) Air in the system
(3) Rear brakes not adjusted properly or not assembled properly.

Just a thought, are the bleeder screws on the calipers on the TOP of the calipers? If they were put on the wrong sides, they would be upside down, the bleeder screws would be on the bottom, and they would never, ever bleed correctly.

And which hole on the pedal is the booster rod? Maybe try the top one.

I've seen weird problems like this due to boogered prop valves.

You say the fronts don't see pressure until the final travel of the pedal? That to me sounds like you've got the front & rear lines mixed up, either at the master or at the prop valve.

Hitting the pedal fast and getting better pressure sounds like a booster issue.

Ditch the prop valve, use a 'T' for the fronts, and put a adjustable in for the rears. There's just no reason to use the old style prop / dist block valves anymore. No worry about it hanging, and you control the distribution..

If you see air, keep going. The driver front is the shortest distance, you should be able to bleed that one easy and quick.

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1) certainly sounds like a leak, but I haven't been able to find it. I've checked and rechecked every connection. There's also no fluid on the floor, and the level in the m/c reservoir doesn't go down.

2) I spoke with someone at inline tubes today, and that's what he thought. I asked him if he thought it could be the combination valve, and he said he thought it sounded like air. He said I need to bleed, let it sit, and bleed some more. I'll spend the next few days seeing if that helps at all.

3) I'm pretty sure that the rear brakes are assembled properly. I've rebuilt drum brakes before. The parking brake works, and I can hear them actuating when I press the pedal - also, the pressure is still low in the back, and assembly wouldn't cause that.

The bleeder screws are at the top of the calipers. I've read enough forum posts to know not to make that mistake :D


The booster rod is currently on the lower hole, which I thought was correct. Within a half an inch of pedal travel, I can feel the M/C piston being depressed, so I think that's all correct. Is it possible that I have it in the wrong hole? Is the upper hole always for manual brakes?

Do you think a brand new prop valve could cause this?

I'm pretty sure that the front/rear lines are correct - as much as I'd like it to be something easy like that. The port closest to the firewall goes to the inlet on the top to the right (facing the firewall) of the combo valve, the port closest to the front of the car goes to the inlet on the top and to the left of the combo valve (facing the firewall).

I keep coming back to the booster - because I recall the pedal feeling different before I replaced the booster, but I stupidly replaced the M/C and something else (I can't remember now) at the same time as the booster, so I wasn't sure which caused the change. Could a bad booster cause an issue like this? It seems to function properly otherwise - the pedal is hard when it isn't getting vacuum, and light when it is (but still softer than I'd expect in both situations).

I might end up ditching the combination valve for a setup like that if lots of bleeding doesn't help. I figured if I replaced everything with the same parts that were used in '78, I wouldn't introduce more variables, and everything would work...


Thanks again for the info - I'll let you guys know how the bleeding goes. I did get some air out of the back right, but not from the back left. (I guess that makes sense if the air was farther upstream?).

I'm not following on this one. Any chance you can post a pic? And yes, a brand new prop valve can be bad very easily. Seen it several times.

The lower hole on the pedal is correct for power, but figured it might be worth a try. Leave it if that's the way it's always been, and look elsewhere for a 'problem'.

You're still getting air? Can't really go much further in troubleshooting until all the air is bled out. Could still be your culprit...

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