First time posting, haven't seen any posts about my specific problem so here goes. I recently replaced my master cylinder because the brakes were barely stopping the car. Ever since then though, they've been locking up on my and staying locked up. I changed a rear caliper and now the rears seem to be ok but the fronts still lock up and stay that way. Once i bleed them a bit they free up, but after a quick test drive around the block the fronts lock up again. I just replace the two front brake hoses but that didn't help. I'm not really sure what else would cause just the fronts to lock up unless it has something to do with the ABS?
Front brakes are locking up
- Thread starter 89TurboLover
- Start date
I don't think it's the caliper. As soon as I let pressure out of the lines they open right up. Somehow it's locking up hydraulically.
Check your booster adjustment. If you pull off the vacuum hose to the booster and the brakes let go, that's the problem. Did you get a master cylinder for a car with ABS? The one without is different.
There's also a master cylinder to brake booster adjustment that needs to be zero'd out.
If its too far out it will hold the master cylinder slightly on and when the fluid heats up it will lock 'em up. Too loose and the pedal feels like its got air in the system.
Sadly you need to remove the master to check and adjust it, but I hink that's what is going on.
If its too far out it will hold the master cylinder slightly on and when the fluid heats up it will lock 'em up. Too loose and the pedal feels like its got air in the system.
Sadly you need to remove the master to check and adjust it, but I hink that's what is going on.
Ok so I adjusted the push rod on the booster all the way and it didn't help. I even pulled the master cylinder away from the booster and it wouldn't release the brakes without me opening the hydraulic line. Could the new master cylinder be bad? Or could it have to do with the valve between the master and the abs unit? Could be a proportioning valve maybe? The diagram in my repair manual just calls it "P & BV" and doesn't have any write up on it.
"Proportioning and balancing valve"
They are rather rare to fail, but not unheard of. Try this: when you get the brakes locked up, crack the lines at the master cylinder (not at the caliper) open and see if the braes release. If the master cylinder is fully released and is still holding pressure its either not adjusted correctly or the master is faulty.
Easy test at that point would be to unbolt the master from the booster with all the lines attached and pull it away a little bit- if they unlock its either the adjustment is still too far out or the master was not designed/manufacturered correctly.
Plan-of-action:
1- Get them to lock up
2- crack both lines at master and see if they release
If they don't release, you have a problem *after* the master, see #3.
If they do release, your problem is with the master cylinder whether it be an adjustment or an internal fault. go to #4
3- Report back here and we'll try to help out on how to trace the circuit
4- Re-tighten all lines and get them to lock again
5- Unbolt the master cylinder from the booster and see if they release.
If they release now, the problem is the booster-master cylinder adjustment- this may require more adjustment or master cylinder measuring and possible replacement.
If they don't release this confirms a faulty master cylinder, replace it!
They are rather rare to fail, but not unheard of. Try this: when you get the brakes locked up, crack the lines at the master cylinder (not at the caliper) open and see if the braes release. If the master cylinder is fully released and is still holding pressure its either not adjusted correctly or the master is faulty.
Easy test at that point would be to unbolt the master from the booster with all the lines attached and pull it away a little bit- if they unlock its either the adjustment is still too far out or the master was not designed/manufacturered correctly.
Plan-of-action:
1- Get them to lock up
2- crack both lines at master and see if they release
If they don't release, you have a problem *after* the master, see #3.
If they do release, your problem is with the master cylinder whether it be an adjustment or an internal fault. go to #4
3- Report back here and we'll try to help out on how to trace the circuit
4- Re-tighten all lines and get them to lock again
5- Unbolt the master cylinder from the booster and see if they release.
If they release now, the problem is the booster-master cylinder adjustment- this may require more adjustment or master cylinder measuring and possible replacement.
If they don't release this confirms a faulty master cylinder, replace it!
Thanks for such an in depth plan of action! That's actually exactly what i spent my afternoon doing. According to my sources and your post it seems like my master cylinder was faulty. When i unbolted it from the booster the brakes didn't release. The parts store said my new master should be in first thing in the morning so i'll swap that out and report back with the results.
Abs units go bad in the open position meaning that they will no longer work like abs but just like non abs cars. So if you hit the breaks hard enough you will just lock up the wheels. Just to let you know.
Well I changed the master cylinder again and sure enough the brakes aren't locking up anymore. Now I can't get it to seal to the booster though. The old gasket was junk so i made a new one with silicone and still nothing. Without any vacuum I don't have any brakes. I'm not really sure what to do now since the gasket i made doesn't seem to be helping.
YOu will have brakes no matter if you have vecuum or not- just not boosted brakes. If you are losing your vacuum reservoir after shutting your car off, then either the gasket or the check valve have failed.
Congrats on getting the brakes sorted!
Congrats on getting the brakes sorted!
Well yeah I have brakes, but not enough to be safely drivable. I am glad they're not locking up anymore but I hope I can get everything else sorted out soon.
89TurboLover;1724854 said:
There's no reason for the MC to seal against the booster. Toyota used a thin gasket that's basically a piece of paper which doesn't really do anything. The booster should hold vacuum by itself even out of the car.
How are the big vacuum hoses to the booster? They get old and stiff and don't seal anymore. And, verify the one-way valve is still one-way-ing. It's that metal cylinder screwed to the firewall near the booster between the two hoses.
Reviving this thread because I am experiencing the same thing. New ss lines, and the brake system was flushed. When It would get hot under the hood they start to lock up but only in the front. So I am at step 5 and when I pulled the MC away they released. So I am going to shorten the rod and see if that helps.
toyotanos;1724022 said:
So I shortened the rod and that seemed to make it worse, which makes no sense. Then I pulled the master cylinder away and it didn't release. So I am thinking now its a bad MC.
Dude I bought a master cylinder at autozone or reilly's and the rear brake plunger must have been installed backwards. Trying to bleed I would have to open the bleeder (rear) for the pedal to come back up. Returned that junk and got a new OEM at the dealer and never had a problem.
Maybe you're buying the junk econo-brake pads and your stopping distance is affected ?? I put those on a sentra once and HAD to put better ones on they sucked so bad.
Like mentioned - Is the brake pedal easier to apply when the engine is idling ? And when you shut the engine off are they harder to press ? Thats your vacuum assist.
Maybe you're buying the junk econo-brake pads and your stopping distance is affected ?? I put those on a sentra once and HAD to put better ones on they sucked so bad.
Like mentioned - Is the brake pedal easier to apply when the engine is idling ? And when you shut the engine off are they harder to press ? Thats your vacuum assist.
I pulled the MC and it did have some goop in it. I cleaned it real well and bench bled it before reinstalling. Bled the brakes again and it still happens. I guess my next step is to actually replace the MC. Kinda bummed so I started to put the interior back in. Looks good even if I can only drive it for five minutes..lol