Brake Problems 1992 Ford F-150

I have a 92 ford truck and the problem is my brakes keep locking up and then about two hours later the truck will drive like nothing is wrong and lately the brakes seem to lock more often i've checked lines and brake pads and when u turn the engine off you hear I low sound it sounds like air coming out of the vacuum pump but im not sure what it is also there is 200,249 miles on the truck itself and about 7500 miles on the engine