Chevrolet: 78 corvette brakes, corvette brakes, 78 corvette

Question
I acquired a 78 corvette that had been stored for 12 years, moisture and old brake fluid in the master cyl.  I have changed out the vac booster, master cyl, proportion valve, calipers and pads.  Installed new fluid, power bleed system 20+ times and has a good hard pedal.  Start up engine and brake pedal fades to the floor.  Re bleed system, no air in system, tap on calipers while bleeding to release bubbles.  No bubbles, no fluid leaks.  What next?

Answer
 OK I think it sounds as there might be a vacume line or
some hose open as when you start it up it is getting air or no power assist to the brake pedal , did you put a new
vac. booster ? or if not you have to have something open  
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if you took the booster off did you plug all lines ?? also I'am not shure but you might have to change the master to a std. one -if you did remove the vacum unit, but if you have a good hard pedal I do not think you would have to. You have a good hard pedal some cars you must have running to bleed the brakes if the unit is hooked up to a power asst. unit, that might be what is wrong ?? you might have to have the car idle and bleed it in park not nutral. as if it needs vacume you have none with it off. ok if I think of anything else I will get back to you, ps let me know if you get it.