[Hotrodjohn71]

Hi group.
After trying everything I have ever known to do as a mechanic for this last week on our 67 Firebird with power drum brakes all the way around to try to get a good high firm pedal after MC changeout,, I finally thrown up my hands to ask for any advice I might get to help solve this brake bleeding problem.
All of the drum brakes have plenty of meat left on the shoes. All drums are new. All of the Springs are in great shape there are no leaks in any of the wheel cylinders or any of the lines anywhere that I can find.
All of the shoes are adjusted to cause a slight drag on all of the wheels. This car has a distribution block (no proportioning valve) with a switch for the brake light which is clean and clear.
I bench bled the master cylinder until there was no bubbles left whatsoever and installed it on the car. I used the tubes-into-the-reservoir method like I have many times before.
After bleeding from the farthest wheel every wheel cylinder (with a partner in the car pushing down the pedal while I release fluid), I still have a pedal that goes down 3/4 of the way before it apples any brakes.
Also, I went to every wheel using my Mighty Vac vacuum tool. Then also I used this vacuum canister you hook up to an air compressor to pull fluid through the brake lines and I used these on all the wheels running through a couple of quarts of brake fluid but still the same result.
I also checked the pushrod from my booster to MC with a piece of clay for a length check and I have about 1/16" play so there's no preload on the MC.
When I had the MC off, I forced brake fluid back through each wheel cylinder bleeder with an oil can connected to a rubber tube I setup to squirt brake fluid, and all were clear indicating all of the rubber hoses were clear. The rubber hoses and all of the steel lines and all of the brake equipment appears to be changed before I acquired the car.
The booster has no leaks and seems to work as intended. I have about 3 good brake applications after shutting off the engine.
. So the car can be driven but the brakes are horrible. Do you have any suggestions before I take this thing to a shop in humiliation? Thank you

Side note. The reason I changed the MC.
I got the car as a project where I was told by previous owner that all of the brake work had been said to be done, and it very much looked that way after inspection.
. After I installed the engine and during engine break-in driving is when i.noticed that after driving a while, the front brakes would begin to drag. They would cause a noticable drag and I would smell brakes and they would be hot, but after parking a while. They would release .
So, not knowing how long the car had been sitting g, I made a guess that the MC was bad and not releasing sometimes.
When I did drive and before the dragging issue began, I had s great high firm pedal.

[RAJ7395]

Just a couple thoughts.

I don't think drum brakes use a proportioning valve but rather a distribution valve. They used different wheel cylinder bore sizes to bias front to rear braking forces. For front disk brake applications, a proportioning valve was added to prevent the rear drum brakes from locking up before the front disk brakes did. Did someone put a proportioning valve in my mistake?

Have you checked to make sure you have the booster input shaft connected to the correct hole on the brake pedal? The upper hole is for manual brakes and gives additional mechanical leverage whereas the power brakes use the lower hole with less mechanical leverage. If connected to the top hole, this would allow the pedal to move a farther down before the power brake booster forces the master cylinder plunger to move.

Just my $.02

[Hotrodjohn71]

Hi RAJ7385,
Thank you for your thoughts.
The distribution block is clean and clear (no proportioning valve)..
And the brake pedal has the pushrod pin in the correct lower hole.
I dont have the wheel cyl part numbers until I pull the wheels off again. I do know however, that they are not leaking. The interesting thing is that these brakes worked like a charm for a while before they went bad.

[FrankieT/A]

Looks you covered everything I would suggest up until this point. So, this is what i would check next. Drum masters are different than disc masters in that they have a check valve in where the line connects, these "checks" keep the shoes out a little bit when the brake is released to keep shoe travel down and maintain a good pedal. You sound experienced so you probably know that. If you are confident that your master is indeed for a drum/drum then the next thing I would do is to clamp off your rubber hoses- either all or one at a time to isolate your four corners(or three as the case may be because you only have one hose at the rear). I have in the past had a wheel cylinder appear to be good and not leak and cause this problem, not saying this is your problem. But those hose trick always provides good info.

Frank

[geeteeohguy]

I never have to bleed the service brakes after replacing the master cyl, as I reverse-bleed them with a syringe. Never had a problem and prefer it to the messy pump and tube method.

[Hotrodjohn71]

Hi Frank, thank you.
Regarding the master cylinder, I obtained a brand new one from O'Reilly Auto Parts. Here is a link to the cylinder.

https://www.oreillyauto.com/detail/c.../ohh0/nmc11372


When I was at o'reilly, I had the counter guy turn his screen around so I could look at what the stats were on the cylinder. It is for power drum drum which is what I need.

It is a 1-inch bore master cylinder with equal reservoirs front and back, similar to the one that I removed from the car, but the one I removed from the car was a square shaped master cylinder which was just a superceded number. A little different looking but perform the same task.

While I am not able to find an actual part number on the old master cylinder, it is interesting how you mentioned that check valve issue.
Because if that old master was the wrong one with that check valve that might explain why the brakes were dragging. But all things being equal, I know that this new master cylinder is the right one, so now it's just about the bleeding.
Would there be any damage to the rubber hoses from clamping them? I can tell you these hoses are not very old because they were changed when all the other break parts were changed.

[geeteeohguy]

Originally Posted by Hotrodjohn71

Hi Frank, thank you.
Regarding the master cylinder, I obtained a brand new one from O'Reilly Auto Parts. Here is a link to the cylinder.

https://www.oreillyauto.com/detail/c.../ohh0/nmc11372


When I was at o'reilly, I had the counter guy turn his screen around so I could look at what the stats were on the cylinder. It is for power drum drum which is what I need.

It is a 1-inch bore master cylinder with equal reservoirs front and back, similar to the one that I removed from the car, but the one I removed from the car was a square shaped master cylinder which was just a superceded number. A little different looking but perform the same task.

While I am not able to find an actual part number on the old master cylinder, it is interesting how you mentioned that check valve issue.
Because if that old master was the wrong one with that check valve that might explain why the brakes were dragging. But all things being equal, I know that this new master cylinder is the right one, so now it's just about the bleeding.
Would there be any damage to the rubber hoses from clamping them? I can tell you these hoses are not very old because they were changed when all the other break parts were changed.

You never, never clamp rubber brake hoses. Huge no-no. It can fracture the inner lining and cause it to seal shut or fail. Always use a plug or cap. I use valve stems through the banjo ends and rubber vacuum caps over the threaded ends or let them drip.

[Hotrodjohn71]

Geeteeohguy,
Are you saying that you have put a dry master cylinder on, then forced fluid up through all four wheel cylinders?
Did you just keep pumping fluid until the bubbles disappeared? And of course keeping the reservoirs from overflowing?

[Joe's Garage]

I'd buy a rebuild kit for the OLD master cylinder - the one you took off - and rebuild it.

Then I'd re-install it and bleed it and see if the problem still exists.

A 2nd trouble-shooting suggestion: Years ago, I redid the brake system on a 1966 Mustang. Non-power drum brakes all the way around. Rebuilt wheel cylinders and master cylinder, new shoes and replaced all rubber hoses.

We could not get it to bleed right until we adjusted the brakes practically all the way out - to the point that the wheels would not turn!

After bleeding, we backed the adjusters off just until the wheels turned and readjusted to get the pedal up to where we wanted it.

Good luck!

[Schurkey]

1. GM quit putting residual check-valves in master cylinders in '71 or '72. If they started putting them back in later, I don't know about it.
2. The residual check-valve did not hold the shoes out. Not even a little bit. If the shoes don't come to rest on the anchor pin, SOMETHING is wrong. Usually, it's the rear shoes that don't rest on the anchor pin, and it's because the park brake cables are adjusted too-tight; or they're seized. There isn't enough pressure from the residual check-valve to overcome spring tension.
3. If the bore of the "new" master is correct, best-guess is that there's still air trapped inside. Really common. Properly bench-bleeding takes WAY more effort than most folks realize. Master cylinders are generally installed on the vehicle with the front tipped up. This causes a bubble of air to be trapped in the front part. Either raise the rear of the vehicle enough to level the master or--better yet--tip the front of the master "down" just a bit; or unbolt the master from the booster leaving the brake tubing attached, then push the front down. Tickle the primary piston with a wood dowel or Phillips screwdriver, you'll probably see trapped air bubbling into the reservoir through the ports.
4. Vacuum bleeding is popular; I quit vacuum bleeding ages ago. I could never tell when I was "done" because the vacuum just pulls air around the bleeder-screw threads--or past the drum brake wheel cylinder cups.
5. Reverse bleeding--injecting fluid into the wheel cylinder to push air back into the master cylinder reservoir--can work OK, but if the vehicle has ABS, the fluid in the system MUST be clean. If the fluid is contaminated--and it usually is--pushing crap through the ABS module can cause the valves to stick or leak. And this is no substitute for having the master cylinder properly bled. A trapped air bubble there may not be pushed out by any bleeding method other than proper bench-bleeding, or dropping the front of the master "down".

[Ram Air IV Jack]

Originally Posted by Schurkey

1. GM quit putting residual check-valves in master cylinders in '71 or '72. If they started putting them back in later, I don't know about it.
2. The residual check-valve did not hold the shoes out. Not even a little bit. If the shoes don't come to rest on the anchor pin, SOMETHING is wrong. Usually, it's the rear shoes that don't rest on the anchor pin, and it's because the park brake cables are adjusted too-tight; or they're seized. There isn't enough pressure from the residual check-valve to overcome spring tension.
3. If the bore of the "new" master is correct, best-guess is that there's still air trapped inside. Really common. Properly bench-bleeding takes WAY more effort than most folks realize. Master cylinders are generally installed on the vehicle with the front tipped up. This causes a bubble of air to be trapped in the front part. Either raise the rear of the vehicle enough to level the master or--better yet--tip the front of the master "down" just a bit; or unbolt the master from the booster leaving the brake tubing attached, then push the front down. Tickle the primary piston with a wood dowel or Phillips screwdriver, you'll probably see trapped air bubbling into the reservoir through the ports.
4. Vacuum bleeding is popular; I quit vacuum bleeding ages ago. I could never tell when I was "done" because the vacuum just pulls air around the bleeder-screw threads--or past the drum brake wheel cylinder cups.
5. Reverse bleeding--injecting fluid into the wheel cylinder to push air back into the master cylinder reservoir--can work OK, but if the vehicle has ABS, the fluid in the system MUST be clean. If the fluid is contaminated--and it usually is--pushing crap through the ABS module can cause the valves to stick or leak. And this is no substitute for having the master cylinder properly bled. A trapped air bubble there may not be pushed out by any bleeding method other than proper bench-bleeding, or dropping the front of the master "down".

I decided to rebuild the original disc brake mc on my 69 GTO several years ago. At that time, you could still buy the Delco rebuild kit. I honed out the mc and the bore looked great. Installed the rebuild kit and began the bench bleeding process following the 69-service manual. I used a Phillips screwdriver as you mentioned. VERY tough however I kept an eye on the fluid level and watched until there were no more bubbles coming up in the reservoir. Bolted the unit on and had a friend watch the fluid level as I sucked all the old fluid out of the brake system. Excellent pedal with no issues. Bench bleeding worked fine but it was tough!!

[geeteeohguy]

Originally Posted by Hotrodjohn71

Geeteeohguy,
Are you saying that you have put a dry master cylinder on, then forced fluid up through all four wheel cylinders?
Did you just keep pumping fluid until the bubbles disappeared? And of course keeping the reservoirs from overflowing?

No. My method is, on the bench or in a vise, fill the MC up about 1/3 of the way. Then fill a syringe with brake fluid, and covering one port with a finger, inject the fluid into the outlet port of the MC slowly until air bubbles disappear and a steady stream of fluid is seen. Then do the other port the same way. Wrap in a rag and install on the car, topping it off after the install. I've been doing this for decades and never have to mess with the wheel cylinders or calipers, as all the air is guaranteed to be out of the MC. This makes less mess than pumping the MC on the bench with tubes or trying a power bleeder. When I do need to bleed the cylinders or calipers after a service replacement or rebuild, I gravity bleed them. Haven't used a power bleeder in years and years.

[kyle_blake]

Are the sizes of the reservoirs in the M/C the same size? It looks like a drum M/C so don't think you got a disc/drum M/C?

Also when bleeding , is your car level? Just an idea that may help?
The cylinders I learned when i posted my OWN brake issue couple weeks back can have couplers in them which act the same as a "residual valve". I think some other manufactures have the valve's attached in series on hard lines opposed to the cylinders without the couplers in them. It appears pontiac, imo, and I'm not sure, only ran the cylinders with the couplers in them? Others would know better than me. For me it wasn't the fix but in my post you will see the links.

[Hotrodjohn71]

Thank you for the input.
Kyle. Yes the reservoirs are the same. I am almost 200% certain I have aquired the correct M/C, and the bleeding has been done in my garage which is very flat.
I took my M/C back to Orilley and got another one (same #). I bench bled it with the tubes going into the reservoir. I bled it with strokes not more than 1" for about 10 mins until no trace of bubbles appeared.
I tapped the M/C. Walked away and came back after a few mins and tried to see if any more bubbles would appear but I could not get any more
I did this with it tilted forward sometimes and tilted back sometimes.
After installing it, I did one thing I had not done before; I had my wife press the pedal while I ctacked the 2 fittings (one at a time) on the M/C while holding a rag up to the fittings.
While she pressed the pedal. I cracked the line(s) but closed them quickly before pedal bottomed out.
I got a good amount of air out of the front port as well as the rear port until after a couple cycles, all that was emerging was fluid.
Next I went to each wheel cylinder and with my wife pushing the pedal, I cracked the bleeder on each wheel and got a little air out of each until all I was getting was fluid.
After all this. I got about a quarter better pedal than.i had before the M/C changeout. So now I have a pedal that goes straight down about half way (instead of 3/4 of the way) before I have braking which quickly applies.
It is now twice as good as before.
So thinking about the possibility of air being in the M/C, but not having time lately to unbolt it from the booster and level it and tickle the bore to possibly cause bubbles to come up, I was in a parking lot with a super steep section.
So the thought occured to me that I might position the back end of the car upwards, causing the M/C to be level then I would just tap the brake pedal, possibly ticking the bubbles up.
So I did this for a long time (engine not running) and when I pushed hard on the pedal to check.progress, I had a great high and hard pedal. This was not booster spring resistance, but a great pedal. The strange thing is that when I rolled off the slant, the brakes went right back to 1/2 travel before application.
So from your advice that it has to be either mechanical over-travel of the hardware, or else air in the system somewhere, all I can think.is that there's still air.
Because I have again removed all of the drums and again inspected all of the hardware, shoe orientation and parking brake cable and system, and finding nothing amis, including all brakes adjusted to cause a drag on.all wheels. And also doing about 20 back-up and brake cycles to set the self adjusters which are all functioning as intended as far as I can check, it might be air.

[kyle_blake]

Yah I feel your pain. Another thing I did is buy plugs for the M/C and try it with car not running and it was hard which "should mean M/C" was fine. I should of started it. When I pulled my M/C there was fluid behind it on booster. So not sure the plugs are the right way of assessing it. Seeing how your pedal is getting harder with more bleeding. Does tend to lead towards a bubble stuck aka air...

I feel you've properly bled the M/C. I feel you went the next step further with wife and bleed it where the lines connect to the M/C properly. She should press down and while fluid is coming out you close up . Which is what you were doing, repeating on both. Is there a bubble still there , down the line?? Is it possible you have to chase that damn thing to whatever cylinder and out??? Maybe? This is what I went through and thought, I'm so done with 2 man bleeding. Give me a power bleeder .

I never ended up with one.

Here is one thing to think about. The distribution block has a rod in it just like a prop valve. when you lose pressure in either front or back the rod moves forcing fluid to the working area.

All over youtube they talk about how that rod must be re-cantered before bleeding, They make a stub block that fits in where electronic switch is so the rod doesn't move while bleeding.

So some guys use compressed air and blow back up one of the brake lines that hopefully push the rod back.

Some guys pull the whole block right out of car and then observer rod and make right. Lots of youtube videos on observing this with prop valves but nothing on our distribution blocks.

For me I removed a brake line and used a pin to move the rod back a tiny amount. I don't know if it had an affect or not. is your brake light on? that would mean the rod has moved. assuming light and bulb and switch works and distribution block internals are not brutal clogging it up.

only other thing that really helped me was building (4) 7-up bottle bleeders and lines. Putting the bottles up high and just pumping the brake myself then going to each wheel and cracking the valve a little bit. In the right order of course.

I also bled my master pressing all the way, I think. Anyway you mentioned 1 inch or so?

Hope this helps?

[geeteeohguy]

Originally Posted by Hotrodjohn71

Thank you for the input.
Kyle. Yes the reservoirs are the same. I am almost 200% certain I have aquired the correct M/C, and the bleeding has been done in my garage which is very flat.
I took my M/C back to Orilley and got another one (same #). I bench bled it with the tubes going into the reservoir. I bled it with strokes not more than 1" for about 10 mins until no trace of bubbles appeared.
I tapped the M/C. Walked away and came back after a few mins and tried to see if any more bubbles would appear but I could not get any more
I did this with it tilted forward sometimes and tilted back sometimes.
After installing it, I did one thing I had not done before; I had my wife press the pedal while I ctacked the 2 fittings (one at a time) on the M/C while holding a rag up to the fittings.
While she pressed the pedal. I cracked the line(s) but closed them quickly before pedal bottomed out.
I got a good amount of air out of the front port as well as the rear port until after a couple cycles, all that was emerging was fluid.
Next I went to each wheel cylinder and with my wife pushing the pedal, I cracked the bleeder on each wheel and got a little air out of each until all I was getting was fluid.
After all this. I got about a quarter better pedal than.i had before the M/C changeout. So now I have a pedal that goes straight down about half way (instead of 3/4 of the way) before I have braking which quickly applies.
It is now twice as good as before.
So thinking about the possibility of air being in the M/C, but not having time lately to unbolt it from the booster and level it and tickle the bore to possibly cause bubbles to come up, I was in a parking lot with a super steep section.
So the thought occured to me that I might position the back end of the car upwards, causing the M/C to be level then I would just tap the brake pedal, possibly ticking the bubbles up.
So I did this for a long time (engine not running) and when I pushed hard on the pedal to check.progress, I had a great high and hard pedal. This was not booster spring resistance, but a great pedal. The strange thing is that when I rolled off the slant, the brakes went right back to 1/2 travel before application.
So from your advice that it has to be either mechanical over-travel of the hardware, or else air in the system somewhere, all I can think.is that there's still air.
Because I have again removed all of the drums and again inspected all of the hardware, shoe orientation and parking brake cable and system, and finding nothing amis, including all brakes adjusted to cause a drag on.all wheels. And also doing about 20 back-up and brake cycles to set the self adjusters which are all functioning as intended as far as I can check, it might be air.

How about taking 5 minutes to disconnect one outlet line at a time and injecting brake fluid into the outlet port of the MC to get the air out? Easily done 'on car' and would fix your issue in minutes. (If you haven't already pumped a bunch of air downstream to the wheels).

[Hotrodjohn71]

Thank you geeteeohguy, I will definitely do that, but I need to wait 2 weeks to get my car back as I just took it to a paint shop.
I