Rear brakes lock up... front disc

I had the same problem. I put in new shoes and springs. Pads and shoes are Firm Feel's metal ceramic. Even with bigger tires they would still lock up. I added a proportioning valve from SS Brakes and dialed it down a half a turn at a time until they stopped locking (3.5 turns total). I spent two days at Watkins Glen and other than over heating them (all four brakes were radiating heat like a blast furnace) they worked fine, never locking up.