I have a 1989 GMC K2500 plow truck w/83k miles on it. A month ago the brakes locked up on the front driver side. I got a new caliper and replaced it. Brakes still locked up. I could not for the life of me figure out what was causing the brakes to lock up just on one side. Thankfully I know several mechanics who told me it was the brake hose. I guess they collapse on the inside and act as a 1 way valve. I replaced the hose today (after soaking the fittings in PB blaster for 2 weeks, which is another thread) and the brakes work great. I am going to change the other side next and the rear has already been done. I thought I would share this with anyone who may have this problem and are as clueless as me.