[Tom Goldman]

Most of the probable issues have been mentioned .
Excessive hose length , bore size ect. Pedal ratio is very important , unless you're an Olympic weight lifter .
Using a larger bore will give more volume but LESS pressure with the same pedal ratio and travel .
With a 6 to 1 pedal ratio and a M/C bore of 1 1/16 to 1 3/32" you should be hitting at least 850-900 # on the first pedal hit .
Have someone pump the pedal while you watch the calipers as they are applied and watch and feel for deflection or movement in the caliper itself and also the rotor.
Proper alignment is extremely important with after market race brakes due to the light weight of the mounting brackets and light weight rotors .
I've seen many brake issues cause by misaligned rear housing ends . Its especially a problem on older GM housing ends that are usually heavily rusted and warped.
Any movement here will make it harder for the brakes to get a full ,positive hold .