Loss of all brake pressure, losing no fluid, bleeding only temp. fix

[hamann318is]

Hey y'all, I'm still having the same sort of brake problem that I posted about earlier.

The story is that after taking the car (91 318is) on the track, I had a very mushy pedal. I took it home, bled it, and all was well... until I took it on to the highway. I lost pressure and had a mushy pedal again. I figured it was the master and purchased a new master cylinder. Bled the brakes, and all was well until I had it on the highway. I lost pressure again.

After consulting the forums, I had a shop bleed the brakes. The same situation resulted. Great pressure... until taking it on the highway where I loose all pressure. I've done this two more times, and nothing seems to resolve the problem.

Simply, the car has a new master cylinder and looses pressure when I have it on the highway. The pressure does not come back; I have a mushy pedal until I bleed the brakes again. More importantly: the car does NOT lose any fluid. There are no leaks; the tank on the MC is still at MAX.

I don't know what is wrong. Could I have a rubbish new master? I don't think it is the brake booster, the pedal is mushy - never rock hard. The pads have decent life left. Any help would be appreciated in solving this problem! Thank you in advance!

[euroshark]

Do all E30s have a vacuum booster? I'm definitely more of an E24 and E28 guru... Check that the hose from the manifold to the booster is not collapsing under the higher vacuum pressure that you are probably experiencing at highway speeds due to higher RPM.

[jlevie]

When you bleed the brakes after one of these occurrences do you get any air out of a caliper? If you do it has to be being sucked in somewhere. My first suspicion would be the master cylinder or one of the seals between the master and the reservoir.

[Aptyp]

Do you have that little valve on the booster hose? Check that valve, or just replace it, even though I doubt that it is the problem.

How about your ABS pump? How does your clutch feel? When you pump the pedal, do you get pressure back? Do you have stock brake hoses, or something else?

[hamann318is]

Do you have that little valve on the booster hose? Check that valve, or just replace it, even though I doubt that it is the problem.

How about your ABS pump? How does your clutch feel? When you pump the pedal, do you get pressure back? Do you have stock brake hoses, or something else?

I do have the valve on the booster hose. I was trying to find the part number on RealOEM, but it wasn't listed. Would you happen to know the number?

I cannot test the ABS right now, as I have a completely mushy pedal. However, the shop told me that they were able to engage ABS right after bleeding the brakes. Typically, when I pump the pedal, I get some pressure, but it takes a few more pumps now (around 5-6) to get any feedback from the pedal. I have some braking pressure when the pedal is almost on the floor but not much.

The brake hoses are all stock. The only thing I have changed on the entire system is new rotors and pads.

When you bleed the brakes after one of these occurrences do you get any air out of a caliper? If you do it has to be being sucked in somewhere. My first suspicion would be the master cylinder or one of the seals between the master and the reservoir.

When I bled it - the first time - there was a good amount of air in the lines. Since then I've only had shops bleed it, and what they tell me is that there is a good amount of junk coming out of the line. I know the seal between the master and the booster is there, and I'm not sure if a fluid leak is the problem. I'm not losing any fluid in the reservoir. I do somehow think that air is finding its way into the lines however...

[jlevie]

If air is finding its way in, as evidenced by a release of air on a bleed, the likely source would be the master cylinder and/or reservoir in the absence of leaks on the high pressure side.

Seeing junk in the fluid during a bleed indicates a pretty dirty brake system. And extended flush would seem to be indicated.

[hamann318is]

If air is finding its way in, as evidenced by a release of air on a bleed, the likely source would be the master cylinder and/or reservoir in the absence of leaks on the high pressure side.

Seeing junk in the fluid during a bleed indicates a pretty dirty brake system. And extended flush would seem to be indicated.

The case seems to be that air is getting into the system, but there is no fluid leak.

What sort of things should I be looking for around the master, or should I just consider buying a new master cylinder (even though this one is new)?

[AndrewBird]

Junk in the lines sounds like the inside of the flex hoses are coming apart. They can deteriorate on the inside without affecting the outside. Wouldn't really explain a mushy pedal though... I suppose they could be swelling under braking, decreasing pressure.

It's possible you got a bad "new" MC. A lot of times "new" means "rebuilt" and they are bad from the start due to improper assembly.

[hamann318is]

Junk in the lines sounds like the inside of the flex hoses are coming apart. They can deteriorate on the inside without affecting the outside. Wouldn't really explain a mushy pedal though... I suppose they could be swelling under braking, decreasing pressure.

It's possible you got a bad "new" MC. A lot of times "new" means "rebuilt" and they are bad from the start due to improper assembly.

I agree that my lines are probably horrid. I'm going to replace them for sure.

Is there anything I can check for in determining whether or not the MC is actually bad? It seems fine until I get it on the highway, where it just goes to hell.

[Aptyp]

34331156299

#13 on http://www.realoem.com/bmw/showparts...03&hg=34&fg=25

I would check the little rubber pads that your brake fluid reservoir sits in on top of the master cylinder.

[DynoTechMotorsports]

i have had this problem on cadilacs turns out to be the ABS pump, but not for sure on the E30

[JSiN]

im syked i just found this thread.... i am experiencing this EXACT thing, mushy brake pedal, and it comes and goes.....and its back.
mine started a few weeks ago, right after my car got towed... i got it out of impound, and right off the bat i noticed my pedal almost hit the floor. the car still stops, but it def doesnt feel too safe... anyways, so after a few days i started messing with the mc and checking the various hoses for any sort of leaks/cracks, and magically it slowly returned to normal....until last night. last night i had jacked the left rear tire up off the ground a bit, just to be able to spin it to turn the engine as i was adjusting my valves, and when i was done, i lowered it, and right after i hi-fived myself for finally getting the valves to stop tapping, i went to drive it and noticed my pedal, AGAIN, almost hit the floor. WTF?!?! so im wondering if it has something to do with the wheel being jacked up, just like when it was towed???
so im also hoping somebody has some light to shed....

[Nick325is]

I had a similar problem. I bled all the brakes and the pedal still went to the floor. Replacing the master cylinder solved the problem. Make sure to bench bleed it first though!


-Nick

[Intro]

What might have happened.......
Brakes had some dirt, rust, gunk deposits in the lines and when you bled the brakes some of this gunk could have gotten passed the brake caliper piston and destroyed/ruined the seal or the cylinder itself. just a thought

[abepark]

pump the brakes before you go in for the night and when you wake up see if the pedals soft if it is the boosters would be the suspect.