The battle continues... after replacing the master cylinder, the two front hoses, and bleeding, (both manually and a p-ressure bleed), after about 40 miles my pedal goes away. What do I do next?

Sounds like you have rotor run-out, which allows air to be drawn into the system while you drive. If the brake pads/pistons move in & out, as they would with a warped, or "crooked" rotor, this sucks air past the seals on the caliper pistons. You can bleed them 'til the cows come home, and have a good, solid pedal sitting still, but after driving for a while, air is back in the system, and the pedal gets spongy.
This has been an issue with the Corvette disc brakes since '65, until the '84 models.

I agree with Joel.  We see that fairly regularly.

Larry

Great, thanks guys. So now I have a plan of attack!

Oh Frap !!  You mean to tell me that I have this to look forward too in my 81??   I had all four wheels dun last year.  Put in SS.

dskopp... I would say you are safe; my original issue began with a dragging caliper; guess it got hotter than I thought

 I put in Mershon stainless steel brakes, took care of the problem,,rd