Low pedal problem

[kendogg]

So, in diagnosing the other issues on this car, I have a low pedal issue. I've visually inspected everything - plenty of pad, no visible leaks, etc. Master cylinder was replaced about a year ago by PO. I can push the brake pedal down 3/4 of the way and it does absolutely nothing. Feels fine when it gets down to where it actually works though (about 1/4" off the floor).

However, I've noticed a few weird things about how the brakes act. First of all, you can almost feel the brakes try to grab when you first depress the pedal, then give up completely till the pedal gets to the floor. Most of the time. ONE TIME, I pressed the brake pedal, and I had good braking action up top where it was supposed to before the pedal fell again.


What am I looking at here? I haven't tried to bleed the system yet, as I have the front end all apart waiting on a new control arm and bushing. Should I bench-bleed the master cylinder first, or just try re-bleeding the whole system? If that fails, master cylinder replacement? I see something odd under the master cylinder, I'll try to get a picture of it to post up. One of the brake lines goes into it. Oh, this is an 87 eta w/ABS. Thanks!!!

[jlevie]

Air in the brake system is a possibility, but that normally just results in a spongy pedal. A flush of the brake system would eliminate the possibility of air. If that doesn't help, replace the master cylinder.

[barry]

i just went through this after putting new brake booster on. after hours of trying to bleed system i needed a new master cylinder. so i took the car and master in hand to roberts bmw in dallas and they flushed the whole system with proper fluids etc and it works fine. its not complicated to bleed the system, its contamination that causes most problems, such as running old fluids through new parts.. also there is lots of lines that need bleeding so it takes time and know how..
gl

[KenC]

Sounds like air in the lines.

Do 3-4 HARD brakes from 50-60mph and get the ABS to engage each time. Air gets stuck in the ABS valves/pump sometimes. Rebleed the system with a pressure bleeder after that, and I'm betting you'll have some bubbles come out.

[kendogg]

Sounds like air in the lines.

Do 3-4 HARD brakes from 50-60mph and get the ABS to engage each time. Air gets stuck in the ABS valves/pump sometimes. Rebleed the system with a pressure bleeder after that, and I'm betting you'll have some bubbles come out.

I can't engage ABS - there isn't enough brake pedal travel to apply the brakes hard enough.

[Scallywagon]

I can't engage ABS - there isn't enough brake pedal travel to apply the brakes hard enough.

I'm having this same symptom

I know that my driver side front had something wrong but I rode it a bit and now it's doing what you're saying

front right slightly engages at 1/2-3/4 and at 3/4 rears engage

Good thing shes a five-speed

I'll be rebuilding my front system tomorrow/this weekend I'll try to post pics and whatever I find

[Scallywagon]

I had brake fluid all over everything first of all, and a missing outer pad, and cocked over-extended piston and severely ground down rotors front and back face

Used a C-clamp to fix the piston finally got it to sit right, threw new pads in...both sides, new rotors, bled the two front caliper piston chambers individually, then bled the system in the correct order, no problems, no air, stock BMW fluid

Aside from the pads needing to seat, brakes are hard and stopping very well

[kendogg]

OK, tried bleeding it today. No go. As I kept pumping, same thing kept happening. Did all 4 corners, pumped almost a quart of fluid thru the system. Sometimes it feels like it has good pressure, sometimes you push the pedal partway down, and it sinks to the floor. Also noticed when the pedal is coming back up, almost feels like it's hanging up on something then snapping back as it gets past it.


Also took this pic. WTF is this thing under the master cylinder?? My other E30 (an 86) didn't have this:

[downernsp]

Bias valve?? or seals between each other in master are bad,so fluid goes in circle.

[kendogg]

Well, I replaced the master with a nice looking Girling unit I pulled from the local yard. So far so good. Bled well, nice firm pedal now. We'll see how it does once the car's on the road after christmas. Thanks for the advice everybody!!

[kendogg]

Just thought I'd update - the master cylinder fixed the problem. I swapped in a master I found a pullapart, and it works great. The pads that're on here just happen to suck, but the hydraulics work fine now, lol

Thanks for the advice guys!!