Rear brakes stay applied....

[Michael_B]

Hi Folks,

I have an issue with my 1991 318ic, and hope someone here can help. The issue is that there is constant presure to the rear brakes. To the point where I could not maintain freeway speeds in 4th gear. I pulled the calipers and rebuilt them both and replaced the pads and rotors. After bleeding the brakes I took it for a test drive and noticed the same issue. I jacked up the car and both rear wheels would not spin freely as they would normally, the fronts however will. I saw that the bentley manual said the check valve could be the problem so I replaced that as well, but the issue persisted. Then I cam across a post leading me to a Grassroots Motorsports article where they had a similar issue in thier spec e30 project. It mentioned that they swapped out the ABS pump, the booster and the master cylinder. This seemed to do the trick for them. I located good ABS pump to swap in and just bled the brakes but I still have the same "dragging" issue as before. The booster appears to be functioning properly and the pedal feels just like it always has. Is it possible that the master cylinder is causing this issue? If I crack the rear circuit up at the front or open the bleeder screw, the pressure is relieved and the wheels spin as they should. It's only when you close up the system and then push the pedal down does it return to being "stuck". I'm hoping someone here will have a suggestion as to where I should look next.

Thanks in advance for any help, Mike

[jlevie]

If cracking the line that feeds the rear brakes at the master cylinder unlocks the rear brakes, I'd say the fault lies in the master cylinder.

[Massive Lee]

I suggest changing the rear lines if they are old. When they wear out, they swollen from the inside and act as one way valves, not allowing pressure to release. It's pretty common.

[Michael_B]

If cracking the line that feeds the rear brakes at the master cylinder unlocks the rear brakes, I'd say the fault lies in the master cylinder.

I took the line off at the abs pump inlet, but that should be the same as taking off at the master....right?

I suggest changing the rear lines if they are old. When they wear out, they swollen from the inside and act as one way valves, not allowing pressure to release. It's pretty common.

I know that all too well, owning 2002's. But to be honest I hadn't considered that in this case.

Thanks you two for your input and if anyone else has a thought on this, please let me know. Once I figure out what it turns out to be, I'll be sure to post the info here.

[jlevie]

That isn'ty the same as removing the line from the master cylinder. If the ABS is where the problem is, you will relieve the pressure by cracking that line. But the same thing will happen if the ABS is good and the problem is at the master cylinder. So see what happens if you crack the line at the master cylinder.

[Michael_B]

That isn'ty the same as removing the line from the master cylinder. If the ABS is where the problem is, you will relieve the pressure by cracking that line. But the same thing will happen if the ABS is good and the problem is at the master cylinder. So see what happens if you crack the line at the master cylinder.

I'll give it a try in the morning and let you know if that relieves the pressure. Thanks again for the help.

[Michael_B]

That isn'ty the same as removing the line from the master cylinder. If the ABS is where the problem is, you will relieve the pressure by cracking that line. But the same thing will happen if the ABS is good and the problem is at the master cylinder. So see what happens if you crack the line at the master cylinder.

I just got a few free minutes and just like magic, the minute I cracked the line at the master cylinder the car rolled free and easy. Have a new one on the way and hopefully I'll be able to get it done before the picnic.

[Michael_B]

Finally had some time this afternoon to install the new master cylinder. After flushing and then bleeding the system, everything works just perfect!