Please help, brakes suck on 72 c/10

[robert shaver]

I have a 72 c/10 that i am about ready to go crazy with. I just got this truck and the brakes have never stopped good since i,v had it. It does not have soft pedal and it doesn't go way down to the floor, feels like it should stop on a dime. But, this thing does not like to stop, it's got power brake booster that is working, i can feel it helping the pedal. I have to stand on these brakes to stop. I have switched master cylinders, no help, I took some of the proportioning valve apart part way, ( took off big nut and spring off end) looked good. I even capped off the port that feeds the rear brakes to see if the front discs were working some and they seem real week. the only other info I can think would be importent is the truck does stop in a straight line no pulling side to side, Right now i,m leaning to a bad p valve, but any help i would love