[RallySnake]

Thank you guys for the replys and ideas.

Bob: Yes, if I open the bleeder fitting on any of the rear (or front) calipers the pedal will go down to the floor. After replacing the rear pads, there is a break-in period before the new pads are fully seated and become fully effective. I can really feel this when braking hard on mountain roads on a rally! I only replace the front pads when a new technology comes along because they wear so slowly. They are huge compared to brakes for other cars that weigh 2450 pounds.

Greg: The XKE rotors are about 5/16" thick (I haven't learned to think metric yet) and the XK6 rotors are about 3/8". I used the XKE one because the offset matched the caliper better. It's unlikely that the small difference in thickness would matter much.

Mike: The M/C linkage is very simple. I used a pedal from a Vega (my doner car) and a rod with a clevis pin into the M/C. The pedal travel doesn't change much unless a bleed valve is open. Then it goes to the floor. I have not removed the rear calipers since rebuilding them about 5 years ago. They are REALLY hard to get to. You have to dissamble the whole rear suspension. But you may well be correct. These calipers are very old and even though I honed the bores before assembling the pistons and new seals, there may be some kind of irregularity in there. It's is hard to understand why all four pistons would stick at the same point, though. When it quits working, there are NO rear brakes.

Richard: The M/C is a simple, single piston 1" bore GM unit as recommended by the MasterPowerBrake people. This is necessary to work with the vacuum powered remote brake booster. I know now that it is a little dangerous to depend on a single M/C, but it has worked well for over 100,000 miles. Maybe it's time to buy one of those Buick electric power master cyclinders and rebuild the whole thing. That would get rid of the big power booster unit in front of the engine and the backup electric vacuum pump under the fender. It would give the additional safety of dual circuits. I am not sure that it would solve the problem as described above, though. From your ideas, the most likely culprit(s) are the rear calipers, themselves. However, any other ideas are welcome.