Repeat: '90 200qa brake problem

[scott miller]

The brake problem:
I replaced the LR caliper, broke and repaired bleed screw in RR caliper,
allowed the reservior to empty in the process.  Refilled with Valvoline
Synpower, bled thoroughly.  After that, the brake pedal felt firm and tight
with no boost (engine off, bad bomb), but firm pressure sent it slowly to
the floor with boost (engine on).

Replaced the master cylinder with new, not rebuilt.  Replaced rebuilt LR
caliper for slight leak in bleed screw.  Bench bled the MC, bled everything
thoroughly, gravity bled, pressure bled, pump bled all lines several times.
Tapped on all components while bleeding, removed calipers and tapped on them
while turning them and bleeding to get any trapped air (none).  The pedal
still feels initially firm, but drops with steady, firm pressure.  There is
a light vibration during the pedal drop, feels like the hyd pump pushing the
booster (bad bomb).  The brakes seem to work well, the rears might be
contributing less, but it's hard to tell.  Turning off the ABS doesn't make
a difference.  No apparent loss of fluid, and no leaks I can find with
careful cleaning and inspection.  I suspect the new MC, which is under
warranty.  If not, I'd like to avoid long and expensive troubleshooting by
my local trusty, but expensive, mechanic.  He's already looked at it a
couple times, no luck yet.

Any ideas welcome at this point, I'm stumped.  If someone out there has an
idea that prevents a big repair bill, my wife *might* let me keep it.

Scott Miller
'90 200qa
Holland, MI

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