I have an '85 3/4 ton (4x4) with the heavy duty brake package. I get the same problem nearly every time I park the truck after driving in the wet. Next time I go to leave I get the rear wheels locking up solid the first one or two times I apply the brakes. I'm usually just pulling out to the drive at the time so no real speed involved but they come on with a real bang and stop the wheels dead. I've got fairly old shoes in there and was thinking that might be a factor. I've already tried replacing the wheel cylinders and the master cylinder is also fairly new. Any help much appreciated. By the way after the first couple of applications everything works fine.