Brake system issue...

[Akimbeau]

I have a B&B cobra with a 4 piston Wilwood front system and the GM calipers with E brakes on the rear. I have a new 1-1/32" bore master cylinder. The issue I'm having is that the pedal will go about half way to the floor on the first press and then will be 2" to 3" higher on subsequent presses. The pedal feels spongy as well. The pedal will stay high until 10 seconds or longer elapse and then the first press again will result in the lower pedal. I've bled the brakes several times with no air bubbles apparent. When I apply the E brake, the pedal becomes super firm and high, suggesting my problem is with the rear. Maybe not? I've replaced all the rubber lines with new SS braided lines, hopefully eliminating any hose flex issues. The rear calipers are in good condition and the E brake levers on the calipers are in proper adjustment. I also installed a new 2lb residual valve in the rear line. I currently have the master cylinder configured with the front brakes connected to the port closest to the end and the back brakes connected to the port closest to the pedal. I have a proportioning valve installed in the rear line, but no adjustments on it affect the pedal issue. Any ideas about what could be causing this issue?