[37Ford]

The power brakes started locking up after sitting a while during a reprint. It would move, but you could feel them sticking. I changed the master cylinder, and it's still doing it, both front and rear. The pedal feels normal. Ideas?

FYI, I have a 37 ford with a corvette style master and four wheel discs.

 

[1971BB427]

Check for freeplay in the rod between pedal and power booster. If there's not enough freeplay your brakes will lock up. I've had them work their way out of adjustment during driving until the brakes drug.

 

[vinniekq2]

pull the wheels and pry back all the calipers.disc brakes dont usually lock up unless they are rusty. make sure you pump up the brake pedal before driving after prying the piston(s) back in calipers,

make sure slides are clean and lubed

 

[37Ford]

I checked the pedal after I installed the new master. I assumed that was the culprit, but it didn't change anything. It's just odd that all of them are sticking. Is it possible that the rebuilt master is bad? The booster? What about the residual valves?

I'll take a closer look at the calipers tomorrow.

 

[cobalt327]

I checked the pedal after I installed the new master. I assumed that was the culprit, but it didn't change anything. It's just odd that all of them are sticking. Is it possible that the rebuilt master is bad? The booster? What about the residual valves?

I'll take a closer look at the calipers tomorrow.

If the MC is for a drum brake system, it'll have too much residual pressure built in.

If you jack it up and open the bleeders, that'll tell you if there's a problem w/residual hydraulic pressure or if the problem is mechanical like stuck caliper pistons or tight wheel bearings.

 

[37Ford]

I used a 69 Corvette 427 style MC, part number 10-1423 (A1 Cardone).

 

[1971BB427]

The newer masters may be different for drums, vs. disc brakes, but not the '69. Chevrolet used the same master for drums and disc setups, but different proportioning valves. They also didn't use different masters for manual vs. power systems.
Do you have a proportioning valve on this syste?

 

[37Ford]

Yes and 2# residual valves.

Can the booster stick? If I take the vacuum off the booster and check if they are still sticking, will that be useful to diagnose anything?

 

[1971BB427]

Yes and 2# residual valves.

Can the booster stick? If I take the vacuum off the booster and check if they are still sticking, will that be useful to diagnose anything?

You'll need to diagnose most issues with vacuum booster hooked up, and engine running for most things. Without the booster the pedal pressure wont be right to figure it out.

 

[37Ford]

Checked the brakes without starting the car or touching the pedal. The whole system was still under pressure and all wheels were locked. The calipers seem fine as far as any rust on the pistons or leaking.

 

[cobalt327]

So if the system was still under pressure, did you crack a bleeder to see if the wheel would free up?

If you have added residual pressure (RP) valves in the brake line(s), you can test them for holding too much pressure by starting it up and depressing the brakes a few times then shut it off.

Next crack a fitting between the MC and the RP valve of the wheel(s) you want to check. That'll relieve the pressure between the MC and RP valve. If there's still too much pressure at the caliper (wheel still won't turn) after loosening the fitting, the added-on residual pressure valve is the problem.

If there's no excessive pressure and the wheel turns freely, the problem could be the residual pressure valve in the MC is faulty (not likely if this has happened using different MCs) or the MC's RP valves are calibrated for a drum brake system, or the MC piston is sticking, etc.

Make sure there's a bit of free play between the MC piston and the pedal push rod w/the pedal at rest.

Be sure the pedal is fully returning after depressing it. Add a return spring if necessary.

The calipers/pistons can look OK but still be sticking. You cannot see what the bore/piston/seal condition is behind the rubber boot. If the brake fluid were contaminated the rubber O-rings could have affected, causing the calipers to seize up/not return after the brakes are released.

 

[37Ford]

Thanks Cobalt. I didn't have much time to fool with it yesterday, but I feel like I have enough info now to diagnose the problem this afternoon.

 

[37Ford]

I cranked the car up and pumped the pedal a few times. I then loosened the line going to the front brakes at the MC. Still nothing. Next, I loosened the line at the residual valve, and what to you know, the brakes freed up. Some pretty nasty fluid came out too. I looked at the valve and tinkered with it a while. I can't really see anything wrong with it, but I'm going to replace it and try to get the bad fluid out of the system.

 

[37Ford]

I managed to take the RP apart and didn't see anything wrong with it. All there is to it is a rubber bit, spring and a plastic retainer. I stuck it back on there since I was going to have to order a new one and it would be a few day. I was hoping it was just clogged. I bled the brakes and gave it another shot.

The front brakes were still freezing up, but not as much as before. The odd thing is that they would lock, but I could just barely tap the pedal and everything would free up. I did this over and over again, and only a little tap would work. I still assume the RP is bad because only the front brakes do this, but it's just hard to imagine it would be that sensitive.