ajg617

My 03 C5 Avant brakes have always been a bear to bleed. This time I'm just not getting it. Just put on the 3rd set of front rotors and pads. They definitely needed to be replaced as the pulsation when stopping was unbearable - and the pads were spent. But, I noticed this time that my brake pedal was getting a bit mushy and I had to depress the pedal further.

Figured new pads and rotors would take care of that - rears still had a good deal left. I did it one wheel at a time on the floor in the garage. I used Pentosin LV which is clear as opposed to the yellow stuff that was in there. I suctioned each one with a MityVac - first time I haven't done it manually with wife or one of the kids pushing the brake until I got the clear fluid with no bubbles.

Good to go right? Nope - mushy pedal. I had evacuated the reservoir at the start to fill with new fluid so it crossed my mind that in some way I pulled too much out and let some air in or by virtue of doing one side at a time, the angle of the car (high on LF for example) left a spot where air could remain in the caliper and not get pushed out.

Took it to a local mechanic and we manually bled all 4 wheels with clear fluid and no bubbles coming out. Pedal mushy as it was. Now normally I would begin to suspect the master cylinder but usually, when that goes, a couple of pumps and the pedal will harden. Not so in this case. With moderate pressure, the pedal goes half way down. Under hard braking I hit the pedal spot which seems to be when the rears put back pressure on the pedal. So I'm still thinking something in the master cylinder with the front brake side but I'm not seeing any leaks at the MC or in any of the calipers. Got me stumped this time.

jcman

Yes it does sound like a master cylinder is causing this, just so you know it is never recommended to bottom out the brake pedal when pump bleeding because the piston seal travel is in an area that causes premature wear and will bypass pressure possibly to the rear.

ajg617

Yeah, I think it's time to replace the master cyl.

Prospeeder

If you do a master be sure to inspect the pushrod in the brake booster. If it can be pulled out, or you find any parts or washers laying the booster. It must be replaced. I have seen this cause impossible brake bleeding.