No brake pressure on front circuit after master cylinder replacement

[pshovest]

Originally Posted by e70jane

.......... It feels like the brakes are being applied while trying to accelerate. The screen will also show dsc 4x4 warning. When I shut it off and back on again, the screen clears and the car drives normally again......

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.

[e70jane]

Originally Posted by andrewwynn

Nope zero chance. It's for sure the boost acting on its own. Checks all the boxes and exactly matches the symptoms.

You should be able to loosen the nuts on the master cylinder to confirm.

It looks like the o-ring seal might be fat enough to perform this test and get the brakes to work properly.


–awr–

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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?

[e70jane]

Originally Posted by pshovest

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.

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.

[andrewwynn]

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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[pshovest]

Originally Posted by e70jane

.........I had to put a lot of pressure on the pedal, continuously, to finally see some fluid come out. But even then, it was just dripping rather than spraying out. I wonder if this is normal?.....

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.

[80stech]

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 ??

[andrewwynn]

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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