Help! '90 200qa brake bleeding

[scott miller]

I replaced a rear caliper, pulled the other rear caliper to fix a broken
bleed screw.  While one caliper was off, the reservoir emptied, air in the
system.  I refilled, and bled the brakes and master cylinder thoroughly.
The brake pedal feels fine with the engine off.  Engine running, the brake
pedal initially gives braking, but with firm pressure it slowly drops, with
a slight vibration apparently from the hyd pump pushing the brake booster
(the bomb is dead).

I=92ve now replaced the new caliper for a slight leak, and replaced the MC
with a new (not rebuilt) one.  Bench bled the MC, bled the whole system
several times including the clutch and MC again.  Still the same problem.
I=92ve gravity bled, pressure bled, and done the two-person pumping method.

Could air be causing this?  ABS (I tried turning it off)?  What?  At this
point my wife is winning the argument against selling the quattro.  I need
help.  I can hold off through the weekend, but it=92s going in the shop Mon=
day
if I haven=92t fixed it.  Thanks

Scott Miller
=9290 200qa
Holland, MI

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