screech

So I got a 86 4runner a few weeks ago. I was driving it around the neighborhood today and the brakes locked up after a few minutes of driving.
I searched and found a few people that say the brake lines internally failed. So I had the brakes lock up again and cracked the lines, fluid was coming out of the flex lines so I know it couldnt be that.
I replaced the vaccum booster a few days ago and that is when I noticed the trouble began. I lined up the brake pedal rod with the previous booster so it shouldnt be the rod length.
master cylinder was replaced last year, so it shouldnt be the master cylinder. replaced the front pads....
anyone have a suggestion as to what I should do next?
should I swap back in the old vaccum booster? reason why I changed it was because the pedal would fall nearly to the floor but have pressure half way down.
I also rebuilt the calipers because one piston was leaking when I swapped pads.
Seriously stumped.... and I am ASE certified in brakes!!! so I feel like a total tard.

screech

One more thing- they locked up yesterday. When I tried it today fully cold they were working fine.

Justinvd

The brake lines swell and create a kinda 1 way valve. They let brake fluid go to the caliper but not back. Another thing ive seen cause brake lock up was the push rod coming out of the booster not being in correct spot. It was kinda off to the side and it created the same problem. Wouldnt release all the way. Hopefully someone who knows more about brakes chimes in.

854x4

Which brakes are locking up the front or back? Also which side left or right or both? Does it just happen or does it happen when you press the brake.

screech

The front brakes lock up. Going back out in a second to jack up the truck see what side.
was both yesterday though. Its the front though that is definatley locking up.

854x4

Does it only do it when you touch the brakes or does it do it for no reason?

screech

only does it after a few good test drives. Went back outside... it started pouring, but got the booster loose, and only put the nuts in a little bit to compensate for a mal adjusted booster. I think its my booster internally leaking because it wants to stall on me whenever the brakes are applied. but no locking brakes at this moment, to wet to get the brake fluid up to temp right now.

854x4

Did you replace the booster with a new or used one?

screech

replaced it with a used one. 30 day warranty.

Justinlhc

Ummm....master cylinder. Had the same problem. Barely made it home one day, had the front brakes smokin hot. Replaced the master cylinder and haven't had a problem since.

854x4

If you didnt have any problems with the brakes before you switched the booster then I'm going to say its the booster, it could be bad or misadjusted. If both front brakes are locking up then its probably not the front lines unless both of them are bad. I dont think one bad line will cause both brakes to lock up. Could also be the master cylinder but make sure you have the booster adjusted before you switch that.

Flash319

I had the same problem. Was the Master not leaking back to the reservoir causing the bouncing of the pads to pump up the lines. I would go about 3 miles and wow.....

Easy test, just pull the line for the fronts off the master and the oil should just keep dripping out. If not then it is NG.

Kiroshu

Your problem with brake lock up is 95% chance faulty master cylinder replace it. If your engine dies down when you hit the brakes at all your booster is leaking and or the vac line going to the booster is leaking, and also needs replaced with a new one or a reman.

screech

So it ended up being the push rod adjustment. actually the entire push rod, and diaphram. I pulled the booster, and the pushrod fell out. tried lining it back up see if could get it back in place for fun. Didnt happen actually the seal in the front was split. I put the old booster in and worked perfect.
thanks for input everyone, I appreciate it.

brtwigg

This is obviously an old thread, but I'm hoping @854x4 is still around and could offer insight on what it means if a brake locks up for no reason. My situation is very similar to this one, and after reading this thread I tried starting my pickup, just letting it run (not touching the brake pedal), and jacking up the front driver's wheel (the culprit) and spinning it from time to time. Sure enough, after about 20 minutes the brake was locked (again, no brake pedal pushing). Thoughts?

akwheeler

Here's the most likely cause:
the master cylinder isn't releasing pressure either due to mis-adjustment or a blocked port inside and the heat of the engine is warming the brake fluid causing a pressure increase.

generally this will cause both front or both rear brakes to drag, but if you have one caliper stiffer than the other it won't lock up or drag until later.

when the brake starts dragging loosen the master cylinder bolts holding it to the booster and see if the brake slacks off. That will indicate that the booster to master cylinder pushrod needs adjustment or your brake pedal pushrod needs adjustment. repeat this test but disconnect the brake pedal instead of loosening the bolts to determine if it's that pushrod.

brtwigg

Thank you, I'll give this a try!

SMD1997

So I got an 07 f150 which has had a recent rotary replacement. Before the replacement it started grinding then Jerking till it pop and I lost my rotary. I replaced it along with a new caliper. But it still keeps acting like the rotary in the back is going to break. What's worse is the back brakes lock up and I can't go anywhere
BTW both rear rotary and brake pads are new. I just need to know what's wrong so I can fix it and drive,, safely

2ToyGuy

I'm lost. Rotary what? Or did you mean the brake rotors, on disc brakes?
If the rear brakes are locking up, and they are disc brakes, chances are a flexible hose's inner liner has collapsed, or partially collapsed, and is holding pressure on the calipers. A good way to check is, as soon as they're locked up, open the bleeder valve. If it frees them up, there's your trouble.
You also need to jack up the rear end, and see if it's just one, or both sides locking up. That can tell you what flex hose might be collapsed internally.

It could also be the master cylinder isn't aligned properly, and when the pedal is released, not moving far enough to release the pressure to the rear brakes. Use the FSM, and check the MC, and brake pedal, alignment. Very important.

Hope this helps a little...
Pat☺

Darkdude187

I have encountered this professionally at several shops I have worked at. There is clear power steering fluid that comes in bottles that look like brake fluid bottles. The give away sympthom is They lock up immobilizing the rig then in about 20-30 mins. they release and you can drive it for shorter periods of time each time. Start by flushing the system with brake cleaner then you may have to replace everything rubber in the system. Since it hasn't been very long you may only need the master. When flushing try to pull fluid back out of the top of the system.