I have had the brake booster, master cylinder, rotors, brake pads, and rear calipers replaced on my 2007 Aspen and when I drive over 55 MPH the brakes begin to lock up without me pressing the brakes. I have to lift the pedal to help release the brakes, but it will tighten up again after a few seconds. I am on my 3rd mechanic right now.

 

My problem was that the brake lines were collapsing in on themselves. Also when flushing the system some nasty crud kept coming out. I had to flush the brakes about 4 times before it was clear. So I replaced the majority of the brake system before I found the problem.