Just for people's info -
I had this happen once too - after replacing a king pin. I did the right side and had to turn the truck around to get enough room to work on the other side. start pulling the truck forward and WHAMMO! pedal to the metal - brake pedal to the floorboard that is....
seems that with all of my pounding with a BFH - i scared the seals right outta the wheel cylinder. either that or just from moving the backing plate around that the old cylinder just gave out.
made me start looking for a dual master....

Build 'em to Drive 'em