Brake expert's please read

BBLM23 said
Pump the pedal and hold it down, crack the line before the prop valve. What do you get?
Pump the pedal and hold it down, crack the line after the prop valve. What do you get?

Maybe prop valve is not all the way open...

Walter gave the best advice: Work your way back and see where your loosing pressure. Most likely it is the proportioning valve.....