Firstly, KenC, I cannot begin to thank you enough! You saved me many more hours of frustration and pointless toil. The problem turned out to be the one suggestion you wrote:

"2) you might have a defectively (or incorrectly) rebuilt caliper -- people have noted this occurence on this list in the past, wherein the wrong halves were assembled so that one half is upside down, and therefore cannot be cleared of an air bubble -- although personally I would imagine that you would have noticed that one of the bleeders is in the wrong place."

Both calipers are assembled with their outer halves reversed! And, since the lower outer bleeders are centered on the caliper body, there is no way to detect the reversal by looking at them unless the pistons are removed to see which cylinder is served by the bleeder.

Another thing about this nasty problem is that whenever the halves are reversed all the bleeders will spout copious amounts of brake fluid, but the outer bleeders will be redundantly bleeding the UPPER cylinders while leaving the lower cylinders full of air and giving the false impression that all the air has been purged.

I confirmed the problem by disconnecting and plugging the front brake hoses one at a time. With the line for the lower cylinders connected, the lower outer bleeders issued no brake fluid. Then, with the upper cylinders connected, the "lower" outer bleeders produced a gusher of brake fluid.

Once again, KenC, thanks. Before you wrote your spot on advice, I was preparing to disconnect a proportioning valve which has seen fifteen northeastern winters. You certainly spared me that bit of fun and games.

Rich