The fact that the problem clears on stop/restart makes it seem more like a limp mode problem then a mechanical issue with brakes. I can't make a case that stop/restart would somehow relieve trapped brake hyd pressure. Are there any other codes?

My '08 3.0si would go into limp mode displaying 4x4, DSC, and other messages, engine revs would jump as trans downshifted. Stop-restart would make everything normal until the next event. My problem was an intermittent crank PS. I found the code w/ISTA. There are many possible causes for intermittent, near complete power loss.

I bled the caliper and abs again, but I haven’t yet driven on the hwy to test if the problem was still there. I did want to point out a strange problem when bleeding the calipers. While bleeding the last caliper, the front driver, loosening the bleed screw and applying the brake pedal, no fluid came out. The pedal was stiff. I had to continuously apply pressure on the pedal to finally see some fluid come out. But even then, it was dripping rather than spraying out. I wonder if this is normal?

Andrew, when you ask me to loosen the mc from the booster to see if the brakes will release, do you mean that once the car starts acting up again, and I am pulled over onto the shoulder, keep the car on and pop open the hood to loosen the bolts?

Yes, it seems like a limp mode condition. However, I never thought limp mode would stop the car from moving. From my experience, it still allows you to drive, but just that power is reduced.

Some malfunction with auto hold could cause similar symptoms if the P brakes applied themselves but if they stopped you from freeway speed you would smell them.

Regarding the master cylinder nuts: if you get the locked up situation, you should be able to loosen them a little bit and it should relieve the pressure.

read some of the threads talking about the push rod being too long and adjusting it. It is exactly the symptoms you describe.

since the problem presented after replacing the master cylinder, it’s probably just a tolerance of manufacture issue.

you can confirm the booster is problem by disconnecting the vacuum line of course plug it with a screw or something and you just don’t have power brakes. You have to push really hard to stop the car, but the problem should go away.

there was a recent thread on XO where somebody had that problem and confirmed it that method


–awr–

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This is not normal. I had a caliper flex hose fail internally that caused this condition. You can loosen MC end of the flex hose and see if flow improves.

It's rare but totally can happen that the brake hose collapses internally.
Back in the day bleed screws could get rusty and cause problems and often we had to take then right out to clean them up to get enough flow to bleed the brakes properly but those days we didn't have rubber plugs on them! ;)

It should be fairly obvious if it's the brakes causing the vehicle to slow down ??

My thought was related. I would have removed the bleed screw to make sure it wasn't plugged/gummed up


–awr–

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I did remove the bleed screw and it was dripping rather than spraying out when I applied the brakes. I wonder if this could also be a potential cause for the problem I am experiencing on the hwy.

Do you have an infrared thermometer?

Weird flow at the caliper might lead to stuck caliper but if just one you will feel it in the wheel it will pull to the side that sticks but also that will get very hot!

You should try driving with the booster disconnected obviously with every needed precaution it will take a lot of force to engage the brakes but it should stop locking up.

Or: loosen the two nuts holding the MC and that should confirm if the booster rod is the issue.

The front left caliper I would consider that part B.

Have you done any of the diagnostic steps to test the booster yet?


–awr–

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You can wait until it acts up or you can do it first to see if you can get it to act up.

The theory is when hot the push rod in the MC is expanding and engaging the booster. The tolerance is very tight like 24/1000 and if they start out touching you will get the exact behavior you are experiencing.

If that fixes the problem the "proper fix" would be to remove the MC, measure the depth into the booster and the length of the rod coming out of the MC and file down the rod until it's the proper length.

Most cars have a threaded rod coming from the booster and and you use a special calibration tool to measure and set it. BMW doesn't they are made one size at the factory.

If this happened to my car i would loosen the nuts to get the rod gap to work by trial and error and then I'd shim the MC away from the booster using shims in numerous places around the base of the MC.


–awr–

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