1976 with a brake problem

I have a 1976 with a brake problem. With the engine off I get a pedal, but as soon as I start the engine the pedal goes to the floor. I have had the brake booster rebuilt, replaced the M/C, replaced the calipers, bleed the brakes, many times, several different ways, and still have the same problem. Any advice would be apricated.

Greetings, I am VERY familiar with that problem. I fought with it for a extended time on a 75'. Cost me a bundle. In the end, we diagnosed it to be air in the system. So...we didn't pump the pedal to bleed. The calipers were sucking air! Two people, one press of pedal, open/close bleeder, one press of pedal, open/close bleeder. Three repetitions of that process, and Done! Good pedal, good brakes. Also worked on a 72' when doing trailing arms. Good pedal, good brakes. One is power, the other is not. Both systems used same procedure. 

I've used a power bleeder with success.  Also Speed Bleeders work.

Here's a link to a post I made many years ago about making a DIY brake bleeder :  Home-made pressure brake bleeder  Scroll down to the end of the thread to see the pictures I posted of it.

At one point I had an issue with the brakes on "The Toy".  Had replaced the master cylinder with a new one and all the calipers with rebuilts from a local Corvette parts house.  Just couldn't get a solid pedal and returned/replaced the calipers, twice.  The parts place wasn't too pleased with this and suggested I check the new master cylinder.  That ended up being the problem - a bad one from the factory.  A second new master cylinder fixed the problem.

The vacuum pump through the bleeder has never worked for me.  Need to be pushing fluid through the lines to the bleeder.