73 Mustang, front disc, rear drum. BOTH rear brakes are locking at barely a tap of the pedal at very slow speeds. Now, when I say locking, I mean locking as if you slam on your brakes at 35mph in daily traffic to avoid an accident. Only this locking is occurring at speeds UNDER 10mph in my own driveway. I'll tell you what I've done so far to it in the last year, but I still have not been able to clear up this problem. NOTE: This problem was happening both before AND after all of the below-mentioned work was done... -Rebuilt rear wheel cylinders -Adjusted rear brakes manually rather than letting them adjust themselves -Replaced master cylinder -Replaced front brake pads -Bled entire brake system THREE times -Jacked car up, rotated tires to check for stopping of the front brakes. They both stopped when pedal was applied. If I think of anything else I've tried, I'll post it as I remember it. It would seem to me that both rear brakes locking up at speeds of less than 10mph at barely a touch would indicate that the front brakes are NOT doing their job, since they do most of the stopping. The real question is, what am I missing? I know I have adequate fluid in the system, there are no leaks, but something is failing. The only two things I haven't checked are a) the front brake calipers (because in my lifetime, I've never had to rebuild a set of those on any car I've ever driven daily) and b) the proportioning valve. *I don't know if that is what it's called, correct me if I'm wrong on that...it's a device that's hooked up in the path just outside of the master cylinder on the driver's side fender well. If anyone has any ideas, I'm open to them. This brake problem is dangerous, I can't even get a proper response out of the brakes in my own driveway, much less out on the street (where I will NOT take it). Thanks, SBTX73351W 73 Grande 351W 4V C4 Trans.