Brake problems

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Joined
Feb 8, 2014
Messages
304
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75
Location
Austin, Texas
My Car
1971 SCJ Green
1971 CJ Pewter
1973 H-Code Copper
1967 Q-Code Galaxie
I have a 73 originally 351 car now has a 429. Vacuum is 11 -12 manifold vacuum. I had the booster and master cylinder rebuilt during the restoration.

Problem is, brake pedal is very easy to push when applied and if I hold the brake pedal down it bleeds off the pressure till it goes to a hard bottom. Thinking I did not have enough vacuum I added a vacuum pump ( very loud ) did not help. I add a vacuum canister and that did not help either.

The manifold vacuum is the same as my 71 J code and those brakes work great.

What do you guys think?

 
Internal leak in the master cylinder. If it were the booster the brake pedal wouldn't be easy to push. To double check the booster, with the engine off pump the brake pedal 3 or 4 times, hold the brake pedal down and start the engine. If the booster is good the brake pedal will go down an inch or so. The vacuum pump should supply enough vacuum even if the booster check valve is bad.

 
Booster check valve was replaced as well. Has to be something internal leaking.



Internal leak in the master cylinder. If it were the booster the brake pedal wouldn't be easy to push. To double check the booster, with the engine off pump the brake pedal 3 or 4 times, hold the brake pedal down and start the engine. If the booster is good the brake pedal will go down an inch or so. The vacuum pump should supply enough vacuum even if the booster check valve is bad.
Ok I tried the test. Engine off, I pumped the brakes about 3-4 times, pedal was firm. When I started the engine pedal went all the way down to the hard stop. I no longer have the vacuum pump on the car because it really did not make any difference and it made way to much noise.

Tim

 
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Perhaps air in the system also.

I wonder if the master was bench bled b4 install?

I would bench bleed master then reinstall it bleed lines at master then get 12-24oz out of

all four corners.

A power bleeding system would be nice to use in this situation.

DOT 3 only fluid

Regards

Paul

 
I have a 73 originally 351 car now has a 429. Vacuum is 11 -12 manifold vacuum. I had the booster and master cylinder rebuilt during the restoration.

Problem is, brake pedal is very easy to push when applied and if I hold the brake pedal down it bleeds off the pressure till it goes to a hard bottom. Thinking I did not have enough vacuum I added a vacuum pump ( very loud ) did not help. I add a vacuum canister and that did not help either.

The manifold vacuum is the same as my 71 J code and those brakes work great.

What do you guys think?
welcome from Latrobe. Myself having the same problem with 4 digit figure in cost beween several shops. Making this a priority issue~same problem with an after market compressor and ...in my 72 into a SUMMIT POWER DISC FRONT AND SSBC REAR POWER DISC CONVESION BRAKES FROM DRUMS. BRINGING UP FACTORS AS MINE PUSHED PEDAL TO FLOOR HAVING NO BRAKES AND LEAKING FITTINGS ......TAKING MINE TO A LOCAL MECHANIC WITH GOOD BACKGROUND IN RESTORING FELLOW MBRS CARS IN THE AACA. WILL GET BACK TO YOU.



I have a 73 originally 351 car now has a 429. Vacuum is 11 -12 manifold vacuum. I had the booster and master cylinder rebuilt during the restoration.

Problem is, brake pedal is very easy to push when applied and if I hold the brake pedal down it bleeds off the pressure till it goes to a hard bottom. Thinking I did not have enough vacuum I added a vacuum pump ( very loud ) did not help. I add a vacuum canister and that did not help either.

The manifold vacuum is the same as my 71 J code and those brakes work great.

What do you guys think?
ps~look into my photo album between pps 7-10 and somewheres in there you will see hard copy of what i am talking about

 
Last edited by a moderator:
Kinda thinking it might be the master cylinder that's bad. But, in the past when the master cylinder was bad I could pump it afew times and get a good pedal.

As it stands I have only one half decent pedal. If I try and pump it it's gone. That's my only concern, is it really the master cylinder or not?

Tim

 
a master cylinder is cheap and easy to swap.

i would buy a replacement master, bench bleed it use dot 3 or dot 4, then install in the car, rebleed the entire system check for leaks and see where you stand, the swap can be done in a few hours at home. there are 2 nuts holding the master to the booster, and just 2 brake lines front and rear. the worst part is dealing with the brake fluid as it will eat the paint off, so you have to be careful to not get the fluid on paint using shop rags, and if you do get a spill clean it up very fast.

what you can do is suck out the old fluid with a turkey baster reducing the amount of fluid that could cause an accident during removal. or just keep the cap on the master when you remove it and when you pop the brake lines jam a rag or rubber vaccum line plug into the hole to keep the fluid in. usually i loosen the brake lines a turn, then pop the nuts on the booster, leave the cap on the master, with the nuts off i pull the master foward to make sure it is free and with the brake lines loose you just open them up quick and pull the master out turning it on its right side so the fittings point up get a rag around it and pull it out.

you can also inspect the vacuum booster seal and push rod, you can additionally also check your vaccum booster check valve on the top. if the reed valve is blown that can cause weird vacuum issues. you can check the valve by pulling it out with a length of hose from the intake vacuum manifold and blow and suck air through the hose, the check valve will only allow air flow in one direction, is you can move air both ways then replace the booster check valve.

the master can be leaking internally causing the pedal to hold pressure some times and then go to the floor. could be a bad oring or mushroom seal inside the piston that gets hung up on something in the bore and it looses pressure.

 
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