I echo what a couple of other people said about the rubber hose swelling on the inside, when you step on the brakes there is enough pressure to force the fluid through the hose, but it won't fully drain back which keeps some pressure on the rear brakes. I had this happen on the front hoses on a low milage '82 Camaro that I bought a few years ago, the hoses looked great on the outside but noth could get through them, fortunately it is an easy and cheap fix.