when you pump the pedal to get pressure, does the pedal stay up or does it fade?if it fades, lok for a leak in the lines , hoses,if the m/c is located on the firewall look behind it, even the floor board inside. if it stays up id say still have air in the system.i worked on a customers vette awhile back. same type problem.pedal stayed up but had to pump the hello out of them. we bleed them brakes so many times.every thing i checked seemed good.filled up the m/c and went to lunch. when we got back i put the m/c top back on, which i left off and went to bleeding one more time.on the last wheel i was geting fluid first the the biggest brake fart i have ever seen and then we had brakes.i would have never thought that amount of air could have been in that system, but there it was or should say there it went.im not saying thats your problem but id start there. four wheel disc can be a real pain in the butt to bleed sometimes.if we can help in any way pm me and i will give you our phone # good luck.