Brakes not releasing after each pedal press

[Weeburn]

Hey y'all, I'm having a bit of a brake issue.

A week or two ago I replaced my booster, after that occasionally when pressing the brake, the pedal would get harder and harder and the brakes would drag more and more until eventually the car was completely stationary, brakes fully locked up. Cracking a bleed screw (any, all 4 wheels are locking up) relieves pressure, the pedal feels normal again.

I re-bled it several times with a pressure bleeder, and also using someone in the seat pumpimg, still did it after driving. Finally I was convinced I needed a master cylinder since it's all 4 wheels (verified this on my lift at work, it locked up on me once as I was pulling it in to work on it), and cracking the bleed screws AT the master relieve pressure, thus eliminating the possibility of the cause being the ABS unit.

After replacing the master cylinder the brakes felt normal (better than before, actually), and I thought the problem was solved until last night, when the car did the same thing, though more gradually.

Any ideas? In the meantime I've just been rolling with my 7mm wrench in the car for just in case.

The only thing I can think of affecting all 4 wheels would be the booster pushrod not being properly aligned from the first time I installed it, but when I replaced the master cylinder I made sure it was installed properly, and until it locks up the pedal feels fine.

[jlevie]

The push rod from the pedal to the booster could be extended too far.

[Weeburn]

Checked the measurement per the Bentley manual, it's set at 125mm as it should be.

I lowered it 10mm last night, it behaved the same. I guess I'll lower it again (and again, if necessary) and hopefully that'll be it. If it never stops after adjustment I guess I'll write it off to a faulty booster, since there's not much else it could be.

By the way I tried pulling the booster hose off when the brakes were locked up, and the brakes released and pedal jumped up a little, so it's definitely a booster-area issue.

[Weeburn]

Update: I've adjusted the pedal pushrod a few times, progressively lowering it since I figure the theory of it being adjusted wrong is that if the pedal is too high, it's effectively lengthening the pushrod, like it's always applying the brakes so pressure can never be released when the brake pedal moves up.

Well it's still locking up, but now I've noticed pulling the booster hose, which always relieved pressure before, no longer relieves it. The car's locked up right now and the booster hose OFF.

Any ideas?

[jlevie]

The only things that occure to me right now are that the booster is mechanically jammed or that the master cylinder is bad. Were the replacement booster and master cylinder used parts?

[Weeburn]

Both parts were new (well, remanufactured), but not OEM. Master Cylinder and booster are both Cardone, from O'reilly, and partsgeek.com respectively.

I'm not suspecting the master itself at this point (not ruling out binding pushrod like you're saying), since it wasn't replaced until the car had already been acting up for two weeks after the booster was replaced, which is when my problems began.

[jlevie]

I agree that the master is the likely likely cause.

[LJ851]

Remove a caliper and try to push the piston back in it's bore. For a hydraulic system to function there needs to be a clear path for the fluid in the system through to the reservoir. When you squeeze the caliper and compress the piston it should push fluid back up into the reservoir.

[Weeburn]

I agree that the master is the likely likely cause.

But the brakes started locking up immediately after I replaced the booster, BEFORE I replaced the master. Are you saying the master was the problem in the first place, and I replaced it with a another faulty master?

[LJ851]

Your symptoms are exactly that of a blocked bleed port in the master cylinder. There are multiple reasons the bleed port can be blocked, compress a caliper to verify wether or not it is blocked. Unless you just want to replace parts, you need to start here.

[Weeburn]

Alright, I'll see if I can get it to lock up on my lift so I can take a wheel off and try to compress it.

[LJ851]

You don't need to get it to lock up, the condition probably exists all the time.

What happens is when you drive the brake fluid gets hot and expands and since it can't back up through the bleed port it applies the brakes which makes the fluid hotter which applies the brakes more which makes the fluid even hotter, etc...