Weird brake issue

[rowsdower]

Recently I noticed my pedal would slowly fall toward the floor if I put constant pressure on it with the engine running. With the engine off, the brake is normal. I get a couple of assisted presses, then the pedal gets hard. But the pedal doesn't sink in this case. The booster is also not leaking down overnight. If I come out in the morning, I still have a couple of assisted presses as if I had just turned the car off.

I replaced the master cylinder (and the calipers, coincidentally), but the problem is still there. So this tells me I have some kind of vacuum / booster issue, or an ABS/DSC issue right?

Originally I bled using a Schwaben pressure bleeder, then this evening I bled using INPA. However, the behavior has not changed.

My vacuum gauge reads 28" Hg on the hose going to the booster. This seems high compared to other vehicles, but I know this car has a vacuum pump for the booster so the pressures may just be higher in this case.

Any ideas on how I can further diagnose this? It does not seem to sync up with any other booster failure symptoms I've read about online. I am hoping this is not a super-expensive ABS/DSC fix...

[02Pilot]

Booster failure will make the pedal hard, not soft, and movement under constant pressure can only occur if hydraulic fluid is leaking past seals either internally or externally. Assuming no visible leaks, something internal is leaking. Having replaced the master and calipers, the only thing left is the ABS pump. Does the ABS function properly? An internal leak is obviously a fault, but if it only occurs in testing conditions, it may not be a critical repair.

[rowsdower]

Thanks, yes I agree that if there is movement in the pedal then there must be fluid flowing somewhere. ABS operates normally.

I drove the car today and it was back to normal. So I'm hoping that there was just an air bubble in the DSC/ABS module or master cylinder that dislodged itself. The only change was that I coded out TPMS yesterday, but that shouldn't be related to the DSC or ABS.

[HP Autosport]

Originally Posted by rowsdower

Recently I noticed my pedal would slowly fall toward the floor if I put constant pressure on it with the engine running. With the engine off, the brake is normal. I get a couple of assisted presses, then the pedal gets hard. But the pedal doesn't sink in this case. The booster is also not leaking down overnight. If I come out in the morning, I still have a couple of assisted presses as if I had just turned the car off.

I replaced the master cylinder (and the calipers, coincidentally), but the problem is still there. So this tells me I have some kind of vacuum / booster issue, or an ABS/DSC issue right?

Originally I bled using a Schwaben pressure bleeder, then this evening I bled using INPA. However, the behavior has not changed.

My vacuum gauge reads 28" Hg on the hose going to the booster. This seems high compared to other vehicles, but I know this car has a vacuum pump for the booster so the pressures may just be higher in this case.

Any ideas on how I can further diagnose this? It does not seem to sync up with any other booster failure symptoms I've read about online. I am hoping this is not a super-expensive ABS/DSC fix...

Sounds like an internal leak within the M/C.

If you are replacing the M/C, make sure you bench bleed it first or you will spend more time bleeding it on the car. Small air bubbles trapped in the M/C or calipers can happen. A few taps with a rubber mallet on the caliper while performing a brake bleed/flush can get the small bubbles dislodged.

[iminhell1]

Get ista/d.
Do a re-calibration of the abs/dsc unit.

That solved all my (more severe than you) issues.
btw, ista is so damn easy to use, native english. totally worth the switch.