Hot Rods Low brake pedal?

Working the final bugs out of the car the last few days. Found the brake pedal very low and spongy so I bled the brake system..like three times. Pedal seems fine when the engine is not running but as soon as I crank her up that all changes. Still seems low but I have a hard pedal 2 inches from the floor. The pedal doesn't pump up higher so I think I have all air out of the system. Have new Willwood discs up front,drums in rear and a new under floor corvette style master cyl with a single diaphraigm booster. Have good vac*** draw from the manifold.
Brakes in the rear are adjusted up so what gives? Bad booster? master cylinder? How can I improve this situation? Any insight would be of help.

 

Low pedal could be:
- insufficient master cylinder or pedal travel
- wrong master cylinder to caliper/wheel cylinder ratio
- incorrect linkage geometry
- missing or malfunctioning residual pressure check valve
- incorrectly adjusted drum brakes

Spongy pedal could be:
- air in the system
- something in the system flexing
- brake shoes different than drum ID
- caliper misaligned with rotor
- defective brake hose
- pads not properly seated in caliper

With the engine running the booster helps you push on the pedal harder and feel more of what's already there.

If you haven't already done this, the first thing I would do is a traditional, one pump at a time, open bleeder - close bleeder, bleeding of the system. Or, a pressure bleed. I that doesn't cure the problem start looking elsewhere.

 

Make sure rear brakes are adjusted. Check residual valves. Should be 2# for front & 10# for rear.
Butch