[Jimbro727]

Thanks for the replies! This one is kind of long because I responded to each individual comment/question individually (I thought it'd be a bit clearer that way)

Originally Posted by PontGuy

I think that the danger of pulling and plugging lines at the combo valve is that if you get air in the system it could give you false readings. My suggestion- hook it all back up, bleed the system and then repeat your test- plug the front of the master and see if you get a hard pedal. But then if you do get a good pedal, reconnect the front and plug the back and see what happens.

I agree, that's my concern as well. The only test I know I can trust is my first one, which was plugging the front port on the M/C (since the system was bled before then, so I know there was no air anywhere). After then, it's a crapshoot, because I exposed the front system to air.

Originally Posted by PontGuy

Also check for continuity at the switch while you do this. If it does not trip the switch then something is wrong there, because you will only be putting pressure on one side of the system.

I did check continuity throughout my tests, and got no continuity between ground and the brake light switch (post) on the combination valve - something is wrong there, right? With one side of the front plugged and the other not, the unequal pressure should have tripped it, right?

Originally Posted by HWYSTR455

The more I think of this, the more I think you should ditch the combo valve.

I've had a bad feeling about this as well. This one is from inline, before this one, I had one from YearOne. Is there any way that I can verify there's something wrong with this valve? Where can I pick up a good one?

Originally Posted by HWYSTR455

Try running a 'T' for the fronts and block the backs, as a test at least, and see how the pedal feels. That would eliminate one variable, and you could isolate it to the booster or master at that point.

I like that idea - where can I get an appropriate T? This is a pretty simple test, and it removes the combo valve, so I can verify everything else works.

Originally Posted by Shiny

Jimbro. Thanks for the update, sorry to hear you are still fighting this, but glad you are back at it because that means the less pleasant priorities are behind you.

Absolutely!

Originally Posted by Shiny

When I went down a similar diagnostic path, I concluded I could not break a line without letting air in and needing to bleed.

If you opened any fittings and did not bleed again, I wouldn't trust any conclusion.

I agree. But then how do I diagnose using this method? Should I bleed between each test?

Originally Posted by Shiny

I'm confused by this. What do you mean by "plugging the front system"? How would that eliminate the rear system? Seems it would indicate rear system ??

I plugged the port on the M/C for the front system, leaving only the rear hooked up. Since I got a rock solid pedal, my thought process was that the rears were not the issue, since removing the front system entirely from the equation produced the desired result (solid pedal).

Originally Posted by Shiny

And I'm not sure about the air in the master. If you plug only one outlet, I'm not sure you should expect a firm pedal. If you plug both, yes, but I am ignorant about the scenario you describe. I did plug both outlets in my master and pedal was solid, so I concluded my master was bled and not "bypassing" as some had suggested. I don't recall pluggin only one, so I just can't comment. I reconnected both outlets and bled again after that.

My thought process was that the pedal in a power brake system with no issues should feel the same with the engine off and everything hooked up as it does with the M/C plugged. By plugging one half of the system (at the M/C) and leaving the other connected, I'm removing an entire system (front or rear) from the equation to try and deduce where the issue is. If I had some magical way of plugging the brake system that didn't introduce air, I should logically be able to plug the front, see the pedal is solid, hook it back up, see it's soft, then work my way down the brake system from the M/C until it's hard again to try and determine where the issue lies.

Originally Posted by Shiny

I think there may also be diagnostic guages available, maybe you could rent one from the parts store ?? It may be a pain, but you could possibly get a direct measure at each wheel and reduce some confusion without breaking fittings at the combo valve ??

I had one for a while, it showed very little pressure at the front wheels. It didn't come with the proper fitting to check the rears. Then I stripped the fitting

Originally Posted by chrisp

Had a similar issue , no signs of leakage anywhere , turned out after i peeled the dust caps back from the wheel cylinders 1 was wet , remember no sign of any leakage . Was such a small leak pushing fluid out and then drawing it back in along with air upon pedal release . Inline tube asian wheel cylinders , got a set from Napa , made in US . wasted 3 days on and off . Been doing breaks for 25 years , 99% of the time when something is leaking you can see it .

Yes I heard about these! I actually replaced my wheel cylinders a few months ago with NAPA units. There was a definite improvement, but not enough, so perhaps it was one of a few issues.

[Shiny]

Jimbro,

I agree. But then how do I diagnose using this method? Should I bleed between each test?

that's what I did.. I think I went through 2 quarts of fluid

....By plugging one half of the system (at the M/C) and leaving the other connected, I'm removing an entire system (front or rear) from the equation to try and deduce where the issue is. If I had some magical way of plugging the brake system that didn't introduce air, I should logically be able to plug the front, see the pedal is solid, hook it back up, see it's soft, then work my way down the brake system from the M/C until it's hard again to try and determine where the issue lies.

I wish I knew more, but somehow the m/c pistons must be independent for front and rear systems. If the front system was solid, and the back was all air, how would you ever feel the back soft effect if the two m/c pistons were tied together rigidly? The front would build pressure, the back would not, but you would only feel the solid front? Make any sense? This is why I am not sure plugging a single port at a time on the m/c is a good test. Maybe somebody with knowledge can clear this up?

Yes I heard about these! I actually replaced my wheel cylinders a few months ago with NAPA units. There was a definite improvement, but not enough, so perhaps it was one of a few issues.

I'll go with this one. I replaced my m/c, didn't fix. Then I found a leak at a fitting, fixed it, but still not right. I can believe you may also have more than one issue.

I still think you should block both ports on the m/c to eliminate it, then bleed, then sit overnight, bleed again, bleed some more...then put pressure on the system and look for leaks. If you don't see any, but the system is still soft, then you either aren't bleeding well or something is leaking. I am totally in agreement with chrisp that the leak could be fluid going out OR air getting sucked back in.

Here's my thought:

after bleeding, push and hold the pedal

if fluid is leaking fluid out, I think the pedal will slowly go to the floor

if it stays firm without moving, then pump it repeatedly

if this makes the pedal go to the floor, and you see no leaks, you are sucking air in somewhere. this is much more likely in the rear system than in the front as the calipers don't move much and the rear shoes can move a lot if no residual pressure is in the line

My combo valve is also from Inline, and I am pretty sure it does not have a check valve for the rear. I say this because:

1. I stuck a paper clip into the port and there was nothing there
2. I added a residual pressure valve to the rear and it made a big improvement.

Someday I will have a reunion with my car and replace the rear cylinders with the NAPAs.

I also never saw my switch trip when I had an imbalance. I am not convinced it works at all.

Good luck

[PontGuy]

I went through more than 2 quarts last year when debugging my brakes. I bled them completely between everything I tried. And I never blocked off ports as part of the debugging process.

I think that Shiny is right about that the pedal should be hard if you block off one of the ports of the M/C even if the other port is wide open, never thought of that.

At the end of the day if the M/C is good and the system is bleeding properly then there just has to be a leak or air in the system somewhere. The reason the the bad rear wheel cylinders are hard to debug is that air can get sucked into them without an apparent leak (unless you have a residual valve in the system.) But I can't think of anywhere else that this can happen.

Interesting that neither of you guys have seen the switch trip as part of the debugging process because it seems to me that it should have. Mine has always been stuck closed and yours is stuck open. Starting to make me think about the quality of the piston assembly in the Inline Tube blocks.

[SS-TA]

One thought on bleeding GM master cylinders: All the GM Disc brake MC I have seen tilt upwards to the front of the car when mounted on the booster. This setup will tend to allow trapped air in the upper corners of the pressure cylinder. It is very hard to remove this trapped air without either removing the master cyllinder and bench bleeding while level OR pressure bleeding through the master cylinder reservoir.

[Shiny]

Pontguy has me wondering again... does the piston for the switch have any other function besides sensing a pressure imbalance? I agree mine should have tripped with all the problems I had. Maybe it just doesn't work well or maybe it takes a wide open system before it actuates?

I would think because of the hold-off valve, that if the rear system was wide open, that switch piston would have to also shut off flow to the rear system or the holdoff valve would never close and the front system would never build pressure.

??? does this make any sense ??

I also am thinking the hold-off function in the combo valve is designed to let pressure build in the rear system before building in the front. This would answer my question about the m/c front and rear behavior. If the rear had a lot of air, the pedal would travel far because it would take a lot of travel to build pressure in the rear system. Then, once it closed the hold-off valve, the front would be allowed to build pressure and the pedal would be solid. This is exactly the way mine acted - after I had sucked air into the rear cylinders, the pedal would go almost to the floor, then grab the fronts.

In contrast, if the front system had air and the rear was solid, the pedal would never travel to the floor because the rear would build pressure and "lock up" the m/c travel before the front built any pressure.

hmmmmm... so Jimbro, could that factor into the diagnosis someway ??

I think this all might mean that if your pedal travels to the floor, the rear system is probably the culprit (or the hold-off valve) and not the front. I don't have any bright ideas about how to diagnose a bad hold-off valve (in the combo valve).

??

SS-TA- That makes sense.. I had a hard time getting my m/c bench bled even when it was sitting flat. It took a lot of strokes before I stopped seeing bubbles, so if trapped by the angle, it would probably be impossible.

[rohrt]

Wow! how old is this thread? I don't think I have the right to complain about a car problem ever again after reading 6 pages of this.

If I were you I would throw in the towle and get absolutly everything brand new and replace it all. Lines, flex lines calipers, rear cylinders, mc, booster.

I fought several break issues with my 68 and the peddal still is not as firm as I would like but no where near what you have gone through.

I don't think you ever mentioned the flex lines? Are they new? Could they be balloning?

I thought I read on team camaro or Pro touring that blocking ports was a good way to seperate the system for trouble shooting.

Sorry I can't be anymore help. I wish you the best of luck. Stay with it and you will be the expert for break problems here on out.

[PontGuy]

Yeah, Shiny and I went through months of this before getting our brakes to work last year and became "experts" of a sort as a result. The biggest learning is that just because you buy new stuff it doesn't mean it is going to work correctly. And sorting out what part is defective when you change a bunch of it at once can be a real challenge.

I looked at the diagram again. Your combo valve is a whole lot more complicated than my 69 block which does not have a built-in hold off or metering function. But I still don't think that either design really blocks off a leaky side. If you look at the diagram you can see that a pressure imbalance will shuttle the piston and trip the switch. But then the piston reaches a hard stop. Once it gets there then the "good" side will build pressure because there is nowhere else for the fluid to go on that side except to the brake cylinders.

Also one fact remains- once both the front and rear systems have been bled there shouldn't be any air in the combo valve. And the fluid has to go somewhere when you push the pedal, so the pedal should get hard as long as there is no air anywhere else and there are no leaks.

Looking at the diagram made me think about my light again. I should be able to pull the switch assembly out and see if the piston is centered or not without losing a lot of fluid. And then check the switch to make sure it is actually working properly- confirm there is no electrical continuity when the plunger is extended.

I am curious about the paperclip trick though. Is the idea to re-center the piston with the paperclip? I don't see how this would work based on what I saw when I pulled the switch out of my original valve.

Walt

[Shiny]

Jimbro - good luck.. keep us updated.

rohrt - I don't think this is really a thread.. I think it is more like onliine group therapy for a few of us with emotional scars from chasing these problems. Over the last couple years, there have been many others that admitted struggling for months to correct similar problems. Walt helped me figure mine out, and I think we both enjoyed the challenge, so you are right - if you want the brakes fixed, all this dialog may not be the fastest path !!


Walt - I'll bet the switch is probably about the last nuisance for you ! After all this time, I would love to hear you got it centered.

The centering tool I saw advertised looks like a small diameter pin. I think the idea is to unscrew the switch, push the pin down onto the v-shaped switch detent groove in the piston, which is supposed to push the piston sideways.

Mine never hung "tripped" like yours, so I don't know if it works.

If you unscrew the switch and don't see the detent groove, then apparently your piston really moved a long ways and this won't work. I think that might be a clue something is broken inside ?

I'd also wonder how you can center it without cracking a line to allow the piston to move against the fluid ?? If the piston doesn't slide when you pull the switch, you might as well pull the whole block and fix it on the bench and bleed one more time.

I don't know if a paper clip would be strong enough, but I doubt it.. you will have lots of room for something stronger if you can see the detent groove through the switch opening.

I'm confident you will figure it out. I'd say pull the switch and go for it !


Good point about the combo diagram and piston. I see what you mean about the piston stops, but it looks like there is a pin off the piston that does something. I still don't understand how the front system could ever get pressure if the rear system was full of air or wide open and doesn't let the hold-off (metering) valve close?

.

[PontGuy]

Follow up on the switch.

As mentioned my 69 block is pretty simple compared to the later combo valve. But I would think that the switch part should be close to the same. Here is my original opened up. It worked fine but looked ancient and I couldn't see using it with all the new SS lines. Also I was concerned about getting the new fittings to seat good.

All that I can see in the switch hole is a spindle that is attached to something on each end (I assume pistons.) And I can see spring coils through both the ports to each side so it looks like the spool assembly is sprung to center.

The part that screws off is not really a switch. Nothing pushes in or out like the sketches show. I put a meter from the threads to the screw head end and there is no continuity, so there is an insulating sleeve through the core. But there is continuity from the screw head end to the electrical connector. And it looks like all that has to happen is for the screw head end to touch a shoulder on either side of the spindle and I would get continuity through the piston and the body of the block, and the lamp would light. And the screw head end itself looks like it may be a stop for the pistons. I haven't pulled the top off the new block yet but I expect to find the spool assembly off-center or maybe a bad insulator sleeve.

Yes the switch is the last nuisance in my brake system, but I am still hard at work restoring the dash and inner firewall stuff. The engine is out and it looks about the same on the other side of the firewall. I have plenty of other issues to resolve...

Walt

[Jimbro727]

Originally Posted by rohrt

If I were you I would throw in the towle and get absolutly everything brand new and replace it all. Lines, flex lines calipers, rear cylinders, mc, booster.

Re-read the thread's title!!

Originally Posted by Shiny

Jimbro - good luck.. keep us updated.

rohrt - I don't think this is really a thread.. I think it is more like onliine group therapy for a few of us with emotional scars from chasing these problems. Over the last couple years, there have been many others that admitted struggling for months to correct similar problems. Walt helped me figure mine out, and I think we both enjoyed the challenge, so you are right - if you want the brakes fixed, all this dialog may not be the fastest path !!

Talk about emotional scars! I shudder when I look at a bottle of brake fluid!


Anyway, I hooked the lines back up that I had disconnected and pressure bled the front. The pedal still feels around the same - harder than it had in the past with the engine off (but still no where near as hard as it should feel - it feels about as hard as it should with the engine ON when the engine is off [no assist]). With the engine running, it's still soft, and I don't get any real braking power until the end of the pedal's travel. I can keep the car from rolling by pressing the pedal 1/4 or a 1/2 down, but that's about it.

I did notice this whooshing sound when I depress the pedal. I had noticed it before, but didn't think much of it. I took a video of it, in case it sets off any red flags for anyone. http://www.youtube.com/watch?v=DJA7MnuZ6dY (May not work just yet, I just uploaded it)

If that doesn't pan out, I think I'm throwing in the towel and taking her to a shop. When I was bleeding the brakes last night, my hands were covered in fluid, and I just can't do it anymore. I think it's time to give a shop a chance - I want to get at least one good summer cruise in this season!! Now to find a decent classic car shop on long island...

[HWYSTR455]

You're going to have a tough time finding a shop that will be able to 'properly' help, you know that right? A classic car shop is going to know MAYBE the ins & outs of a FACTORY setup, and the solution for them I'm guessing will be to shotgun it, replacing parts. That can get expensive.

Give a call to Kore3, bet they can give you some advice, maybe where to look. Or try posting over at pro-touring.com, which is where the Kore3 guys hang out.

http://www.kore3.com/contact_info.php

.

[Ed Fitzgerald]

[QUOTE=Shiny;4070462]Jimbro,



that's what I did.. I think I went through 2 quarts of fluid



I wish I knew more, but somehow the m/c pistons must be independent for front and rear systems. If the front system was solid, and the back was all air, how would you ever feel the back soft effect if the two m/c pistons were tied together rigidly? The front would build pressure, the back would not, but you would only feel the solid front? Make any sense? This is why I am not sure plugging a single port at a time on the m/c is a good test. Maybe somebody with knowledge can clear this up?



The top half of the pedal is for the rear brakes and the bottom half is for the front brakes. Had a company truck (84 Ford Ranger) that I let one of my guys drive and he told me he was having problems with the brakes. Pulled the back drums and sure enough wheel cylinder blown. MC reservoirs (front full of brake fluid ---rear empty). Top half of brake pedal non existent, bottom half would skid front tires on hard application. Replaced wheel cylinder,drum and shoe. Bled system and bled system and bled system to get air out and finally brakes worked good as new.

[Ed Fitzgerald]

Originally Posted by Jimbro727

Re-read the thread's title!!



Talk about emotional scars! I shudder when I look at a bottle of brake fluid!


Anyway, I hooked the lines back up that I had disconnected and pressure bled the front. The pedal still feels around the same - harder than it had in the past with the engine off (but still no where near as hard as it should feel - it feels about as hard as it should with the engine ON when the engine is off [no assist]). With the engine running, it's still soft, and I don't get any real braking power until the end of the pedal's travel. I can keep the car from rolling by pressing the pedal 1/4 or a 1/2 down, but that's about it.

I did notice this whooshing sound when I depress the pedal. I had noticed it before, but didn't think much of it. I took a video of it, in case it sets off any red flags for anyone. http://www.youtube.com/watch?v=DJA7MnuZ6dY (May not work just yet, I just uploaded it)

If that doesn't pan out, I think I'm throwing in the towel and taking her to a shop. When I was bleeding the brakes last night, my hands were covered in fluid, and I just can't do it anymore. I think it's time to give a shop a chance - I want to get at least one good summer cruise in this season!! Now to find a decent classic car shop on long island...

How exactly are you bleeding the system by yourself ? Are you using a vac pump? I know this might sound stupid but if I'm beeding the system with someone else in the car. I tell them to tramp the brakes as hard as possible and as quick as they can and when they pump up a hard pedal have tem hold it and I start at the right rear and break the bleed screw and watch and listen for air. When fluid stops tighten in the bleed screw and repeat procedure. Had a fellow help me one time and he was releasing the pedal every time the pressure went off it and it hit the floor. I just couldn't seem to get the air out and it dawned on me what he was doing. I would also check the rubber boots around the calipers for the O-ring leaking past the caliper piston and filling the boot. Squeeze with your hand and look for wet spots. Definitely sounds like air.

[stucky2947]

I read most of this thread and didn't see anything about the rod that goes in between the power booster and the master cylinder. I had the same problem with the brakes on my '62.

Take the master cylinder off and look at the power booster. There will be a rod (the one that goes thru the firewall and attaches to the clevis pin that is attached to the brake pedal). That rod pushes on the master cylinder piston. There shouldn't be much play in the end of the rod and the master cylinder piston. That was the problem I had. I wouldn't get any brakes until the pedal was almost to the floor. I had replace the ENTIRE brake system, like you did, and couldn't figure for the life of me why I didn't have brakes.

That rod length is important. Consider where the fulcrum point on the brake pedal is and you can see that a small difference in rod length will translate into a large amount of pedal travel. There should be very little 'play' between the power booster rod and the master cylinder piston. Mine had a rod between the two and apparently during re-assembly it didn't get put back. I ended up making one. Measure from the face of the power booster to the rod in it (the one coming from the brake pedal). Then measure from the face of the master cylinder to the end on the master cylinder piston. The rod that was missing from mine was the one that took up this gap (add the two measurement together). I made a rod this long and then added a dimple in the end that contacted the power booster rod. (the rod from my master cylinder had a radius on the end).

I attached a sketch, hopefully it helps.

[PontGuy]

Stucky, you probably just didn't read far enough to get to the rod part of the discussion I remember you asking about yours a while back and glad to hear you got it figured out.

Yes the rod length is important and maybe something we should come back to. When I was debugging mine I found the rod was actually too long so that the M/C was not returning all the way, and I could not even get the system to bleed. It was after addressing this that I eventually found the bad wheel cylinders.

I think we decided the rod length was probably not the problem for Jimbro because his pedal is stopping before it reached the floor. Which brings two questions to mind-

Jimbro, is your pedal reaching a hard stop before it gets to the floor?

If so does that mean the M/C is definitely reaching the end of the stroke, or is there a stop in the booster that would prevent further travel? If there is a stop in the booster then the rod could still be too short.

As for bleeding, I always have used the "wife in the car pumping the brakes" method, similar to Ed. Always worked for me. Given my experience with sucking air in past the seals on the wheel cylinders I personally would not trust vacuum bleeding very much.

And I agree that finding a shop that can fix yours quickly may be a challenge in itself. The brake shops that work on newer cars will probably be just as lost. And most resto shops probably don't have that much brake system expertise. Now if you can find an "old school" brake shop then you might have a shot.

Walt

[Shiny]

Jimbro -

Sorry to hear you are still frustrated.

All I can say is your symptoms are close to what mine were, but maybe not exactly. After I bled mine, I had brakes and they didn't feel too bad. Then, I would drive it around the block, and after I pressed the pedal a few times (10 to 20), my brakes would soften up and I ended up exactly with your symptoms. The pedal would go nearly to the floor before I could get some braking, but I never completely lost all braking effect. I would bleed again and it would be working again, then they would go away after several pedal strokes. I finally concluded I was sucking air into the back system at the wheel cylinders.

Ed - I think you are describing the same thing - when the rears don't build pressure, the pedal travels a lot farther than normal before the fronts start working.

Jimbro, I think you still have air in your rear system and either your bleeding method is not working, you have a leak, you have air trapped in the master, or you too are sucking air on the return stroke. If you can get it to a shop safely and have them pressure bleed, would probably be worth it. If it fixes it, you win. If not, I hope you find someone that can figure it out.

I forgot if your m/c has bleed screws? When I replaced my master (unrewarding repair attempt number 26), I bought one from Inline Tube that had bleed screws. I seemed to have more success bleeding after I got that. If I removed a line between m/c and combo valve, I would bleed at the master first and was surprised how much air I would get there. I figured it was the high point in the system, and air might get trapped there and get pulled back into the master easily. I never understood how bleeding could eliminate a trapped bubble like that, so right or wrong, it made sense to me to have those bleed screws at the master.

Hope you find a simpler path soon.

[Traker82]

How did this turn out, I am having a sililar problem.

[post toastie]

I had something similar with my brakes, it ended up being one of the brakeline was leaking. It was difficult to bleed it kept letting in air because it was a small leak.
A leaking caliper can have a problem. "I have bought brand new or rebuilt calipers fail not hold pressure because the piston does not seal"
Disc brake side of the master is valved differently from the drum side.
The prop valve or the junction where the brakelines distibuted front brakes to back brakes is for the warning light to tell you when your brakepressure is low.

[PontGuy]

Wow, can't believe this thread came back. But a good reference for anyone out there with brake problems.