Rear brakes locking up

[S62]

Well this has happened to me a couple of times now and its to much of a hazard to drive the car.
Its happened about 4 times now. When I'm driving the rear brakes slowly start to engage without me pressing the peda then they lock up and I cant move the car. When this happens I just leave the car and wait a couple hours and they are back to normal (thank god it has only happened around town close to my house).
Any ideas on what could be the problem? A bad brake booster?

[Wschnitz]

Are they drums, tell us year, and model of your car. Might be a bad brake fluid pump.

[blunttech]

Well this has happened to me a couple of times now and its to much of a hazard to drive the car.
Its happened about 4 times now. When I'm driving the rear brakes slowly start to engage without me pressing the peda then they lock up and I cant move the car. When this happens I just leave the car and wait a couple hours and they are back to normal (thank god it has only happened around town close to my house).
Any ideas on what could be the problem? A bad brake booster?

more info. Is this a swap car with a modified booster rod?

[S62]

Are they drums, tell us year, and model of your car. Might be a bad brake fluid pump.

No they are discs. Its a 87 325is. Are there individual pumps for the front and rear. Someone told me that the brake booster has 2 pistons in it, 1 for the front and 1 for the rear. So one of them may be going bad? The car has a shit ton of miles on it.

[S62]

more info. Is this a swap car with a modified booster rod?

No the car has a stock m20 in it with the oem booster.

[BrownBeanE30]

its ur brake booster cause i had the same thing happen to me i just took the lil rod out and shaved it down a bit and it fixed the problem hope its the same for you and nothing bad

[blunttech]

I dont understand if nothings been done to the car why it would start now. Ive seen this happen with the incorrect push rod length after a swap. The rears drag slightly and expand as they get hot. My guess is a stuck caliper in the rear or collapsed rear rubber lines not allowing the calipers to release.

[S62]

I dont understand if nothings been done to the car why it would start now. Ive seen this happen with the incorrect push rod length after a swap. The rears drag slightly and expand as they get hot. My guess is a stuck caliper in the rear or collapsed rear rubber lines not allowing the calipers to release.

Already replaced all the brake lines

[immajackuup]

rear brake suppose to drag and if the rear is lockingup which mean piston are seize.

[Jean]

Booster does not have pistons, MC does that job.

[der affe]

I was just about to post that ^^^^

[Vtec?lol]

are you sure it's the brakes?

Sounds similar to bearings.

Both wheels or one?

any sound at all?

Are the rotors hot?

Put the rear on jack stands and let the wheels spin a bit while listening. look and feel the rotors but don't apply the brakes, just let the wheels come to a stop. If the rotors are cool to the touch, it's you're bearings.

[der affe]

As stupid as it may sound, what fluid are you using? Dot3 can break down the seals and swell them making the brakes stick. A long shot, but something that gets overlooked, especially when people are comming from an american car back ground to european cars.

Dot 4 is what you should be using as a basic brake fluid.

[Michael_B]

I had the exact same thing happen with my 91 318ic. In my case it turned out to be a bad master cylinder. I initially thought it was my calipers and rebuilt them. I also swapped in a (known good) abs pump and a new check valve at the same time. Once I bled the system, everything appeared to be working fine. I took the car out for a test drive and after a couple of miles the rear brakes started locking up again. When I pulled over I decided to crack the line to the rear brakes, at the master cylinder. The pressure released and I was able to limp back home. Ordered a new master and after installing it and bleeding the entire system again (thank you motive bleeder), everything was ship shape. Hope this helps.

[S62]

are you sure it's the brakes?

Sounds similar to bearings.

Both wheels or one?

any sound at all?

Are the rotors hot?

Put the rear on jack stands and let the wheels spin a bit while listening. look and feel the rotors but don't apply the brakes, just let the wheels come to a stop. If the rotors are cool to the touch, it's you're bearings.

kinda a stupid question but should I do this with the car on or off? The brakes dont permanetley lock up they unlock after a couple hours. So either way if I were to do this test the rotors would be cool to the touch. Tell me if that makes sense.

As stupid as it may sound, what fluid are you using? Dot3 can break down the seals and swell them making the brakes stick. A long shot, but something that gets overlooked, especially when people are comming from an american car back ground to european cars.

Dot 4 is what you should be using as a basic brake fluid.

Im using dot 4

I had the exact same thing happen with my 91 318ic. In my case it turned out to be a bad master cylinder. I initially thought it was my calipers and rebuilt them. I also swapped in a (known good) abs pump and a new check valve at the same time. Once I bled the system, everything appeared to be working fine. I took the car out for a test drive and after a couple of miles the rear brakes started locking up again. When I pulled over I decided to crack the line to the rear brakes, at the master cylinder. The pressure released and I was able to limp back home. Ordered a new master and after installing it and bleeding the entire system again (thank you motive bleeder), everything was ship shape. Hope this helps.

Thanks, good to know. And yes the motive bleeder is pretty sweet.




So after reading everyones responses it sounds like it could either be the master cylinder or the bearings. Or possibly the brake booster like BrownBeanE30 posted early.