Thread: Brake problems

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Hello,

I recently went for my first test drive on my 32 roadster, and as expected I have to work out a few kinks. The car was running fine and everything seemed to be working well, when the front brakes started to lock up. The brakes worked fine when the car was cool, but once it started to heat up the front brakes started to lock up. The hotter it got the worse it got. By the time I made it home from a 5 minute drive, the rotors were smoking and the front was completely locked. I loosened the bleader, on one side and that immediatly released the brakes. My setup is as follows. Non power master cylinder under the floor, disc up front drum in the rear. 2lb residual valve to the front, 10lb to the rear. standard old school 70's master cylinder, and gm proportinate valve in the line. I've been scratching my head over this, I realize that the fluid probably expands once it gets hot, but not sure why it would lock up the brakes, any suggestions would be helpful. thanks

Since you have a proportioning valve I dont think the extra residual valves would be needed as they should be built in,you might want to check the adjustment of the master cylinder. Jeff

Yes, check the adjustment on the rod to master cylinder so there is a an 1" of free play at foot pedal. Also make sure your master cylinder is compatable with disc brakes with no residual ball or valve built in. The only other thing I can think of would be that brake line is to close to the exhaust and is heating up and expanding, this would be remote though. Lon

Come to think of it, if it's the adjustment to the master Cyl. you would think that both fronts would lock up.................. Lon

I agree with Jeff, it is the residual valve keeping pressure on the front end. Same thing happened to my Kid's T, front brakes started dragging after a short drive. We pulled the guts out of the front residual valve so it is now just an empty connection for the two lines, and the problem went away.

They tell me some master cylinders already have a residual valve built into them, so maybe the doubling up was the issue. I bet if you take a drill bit and run it down through the front residual valve like we did, you will be fine.

Don

Sorry I re-read your first post and see it is both fronts locking up. So it still could be the master Cyl. adjustment among other things. A proportioning valve can be just that OR a proportioning valve and residual all in one together. Like already said I don't see why you need to duplicate the residual part. Still would check master cyl. and make sure it's for disc drums with no resid built in, I have been sent the wrong one on several occasions. Lon

I agree, it's the master cylinder. Check that the linkage is allowing the piston to return fully when released. If it doesn't, the line pressure won't release.

You should have about a 1/16 or 1/8 inch of play in the rod that goes between the master cyl and pedal to account for expansion in the fluid.

Another thought, is the brake pedal returning completely to a relaxed position or is it possible it's leaving pressure on the master? On my truck I needed to install a return spring. I do have a booster but the weight of the pedal assembly kept just enough pressure on the master cylinder to cause the brakes to drag....Just a thought

Hi guys thanks for the replys. At first I thought my problem was the rod going into the cylinder but I don't think that's the problem now. I did some playing around today, and I think that the problem is the residual valve. Perhaps my master cylinder already has one built in. today i got the brakes to the point were they were locking up, I then opened the brake line up after the residual valve going back to the master, the pressure never released at the calipers. I then reconnected blead the lines got them brakes locked up again and opened up the line just before the residual valve, and long behold the wheels spun freely again. I'm convinced that it's the residual valve holding to much pressure in the line. tomorrow I will rerun a temp line eliminating the residual valve and see what happens. I like the idea of just drilling out the internals of the residual valve rather than rebending/crimping new lines. thanks for the advice I'll let you know how it goes.

Yep, your problem sounded exactly like what we had going on. After gutting the residual valve the problem went away. We did the same test you did, and found there was still pressure at the wheel when the brake was not being applied. To be honest, I'm not sure most applications even need one. I know they are supposed to be used where the master cylinder is lower than the wheel cylinders to keep the fluid from draining back, but ours are working fine now without one.

Don