brake question (pedal to the floor)

[exmonkeypunk]

I'm pulling my 72' GMC C/2500 into the gas station when my brake pedal goes to the floor and my brake light pops on. I swerved to miss the parked car at the other side of the pump and wiped out a few cones laying around before i was able to coax the 3/4 ton beast to a stop. I looked under at all the wheels and around the lines and saw nothing. I popped the hood to check the fluid level and there was a bunch of brake fluid that spewed from under the master cap. The levels were still full. I dried off the master top and put it back on. Still had no pressure. I have a front disc rear drum truck. Does this sound like a bad proportioning valve? I replaced my right rear wheel cylinder a few months ago but haven't checked the left, but haven't seen any fluid around the drum.