stolarzj

Sorry to hear about Baker, at least you left..

I tried shortening rod by about three turns of the buckle and it had no effect. After a number of brakings to warm up the system the brakes locked up and pulling back on the pedal freed it up. I think I'm going to try putting my old MC back on and see what it does. I just need to try to source some ATE Super Blue locally instead of waiting for UPS.

Jim bailey - 928 International

The brake booster has two chambers inside , one on each side of the diaphram. When the engine is running there is vacuum on BOTH sides. Press down on the pedal and there is a valve which allows outside air to dilute the vacuum on one side of the diaphram....the vacuum on the other side helps depress the rod into the master cylinder. Lift your foot and the valve is SUPPOSED to close and the vacuum equalizes on both sides of the diaphram and there is no push on the master cylinder. If the valve is faulty there can be pressure on the master cylinder. Obviously cracking the brake fluid line will release the brakes but that was NOT the problem. A lack of vacuum would mean that there would not be enough on either side of the diaphram so no lock up . If the booster is infact pushing down when it should not be that would mean that the brake fluid return port would be blocked by the piston and the brake drag lock up would get worse as the brakes got hotter. By pulling the vacuum line off the booster you can CAREFULLY drive around and see if the brakes lock up. Odds are they will NOT. Note that without the brake assist it takes much pedal pressure to stop the car.

insite

another thought to ponder: the EPDM square profile piston seals in a brake system have two jobs. one is to prevent fluid from leaking past the pistons or air from leaking in. the other is to retract the pistons when the pedal is released.

as EPDM ages, it softens; it's durometer decays. as this occurs, the seal's ability to retract the piston decreases. this can lead to pads dragging a bit. this in turn causes heat buildup, which can ALSO soften the EPDM. the result is a positive feedback loop, whereby the seals are essentially destroyed and the pistons no longer retract. replacement of the seals in this type of scenario would solve the problem.

on another note, it would be peculiar for this to happen at all four wheels at once. usually, it only happens on one wheel or one axle. just a thought.

a4sfed928

Try Autometrics on Belgrade Ave they should have ATE Super Blue. You could try Jim's solution in a different way by duplicating the problem then disconnect the vacuum one way valve to release the vacuum from the booster to see if the brakes release. Before you reinstall the old Master Cylinder look back in the booster at the rod to see if it has a wrench flat on the rod. If so try adjusting it before you go back with the old MC.

a4sfed928

My point exactly!

JHowell37

So in this scenario you have described, which component would be at fault? The vacuum booster?

Jim bailey - 928 International

Correct the booster is boosting when it should not be doing so.... if the brake pedal freeplay is OK. Then it is the booster boosting if removing the vacuum line releases the brakes ! This applies when all four brakes are locking/dragging.

insite

water in the booster can cause this

stolarzj

I did wash the engine bay over the weekend, not sure how I would have gotten water in there, it seems pretty sealed up.

stolarzj

I just recreated the problem again and this time I pulled the vacumn hose from the booster. The brakes still remained locked up with no vacumn in the booster, the only way to free them was to pull back on the pedal. So I guess pulling the MC is next and I will swap my old one back in. If your thinking it's a obstruction in the new MC, is there an easy way to pull it apart and clean it out? A new one will take another week to get.

The only problem with the old one was the pedal felt week and spongy with the old one after the brake line change and flush. The new MC did fix that issue it was nice and solid and has nice braking power.

stolarzj

I got the old MC back in and the pedal is hard until I fire up the car and I can push the pedal right to the floor. I only bled it at the MC, not at the wheels, I doubt it's that much air in the system. Either the old one is that bad or I got booster problems now, which I doubt since all I did was swap MC's. Though the when pulling the new MC out it was hanging up coming out and after some pulling I could hear the vacumn escape like there was vacumn inside the rod for the MC. These brakes are getting aggrevating now..

Jim M.

(applies to '86 up cars only) With the engine off, apply the brakes until you hear no more hissing from the booster and the pedal is harder to press. Let it sit for a moment, then press the pedal down gently with your hand. You should have about 10 mm of free play. If not, shorten the pedal push rod a very small amount and recheck the free play. Adjust until you have 10mm free play at the pedal. If you have the brake booster off the car, the center of the hole on the push rod should be 158-160mm from the face of the booster.

Look in the booster. The air INLET for the booster is in the passenger compartment around the pushrod from the brake pedal. If the control valve leaks (the system works by venting air into the backside of the booster and closing off the connection to the front side, which is the vacuum reservoir) you’ll hear a hiss. Verify by turning the engine off, putting a steady pressure on the pedal and listen for the hiss. It should hiss while moving the pedal and then stop. If it continues, the valve in the booster is leaking. A steady hiss doesn't sound like a hydraulic problem unless you find leakage.

Mrmerlin

check the depth of the MC from its mounting surface to the center of the MC piston, compare both masters, since your old one was working the new one should also be the same depth

a4sfed928

Did you pull the check valve? If not there was still vacuum in the system.

stolarzj

I didn't, so that was probably what I heard. I figured just unbolting the MC and seperating it from the booster would break the seal.