Hope some one can shed some light on this new problem with rear brakes????

Yesterday while I was backing out of my drive way, I got halfway down the drive and the vans right rear brakes locked-up. I could not move backwards any further. I then went forwards and tryed again, same thing! so, I tried driving all the way back up my driveway then I tried backing out of my driveway again and low-and-behold, no problems. I drove around and everything was good. No problems after that. The only diffrence I felt was that the brake pedal was and has remained at the top instead of the pedal being about halfway down as is usual. 

The master cylinder is right at the max mark..

This is an '87 Wolfburg that has had no brake problems in the past other then over a year ago the brakes all the way around were replaced.

Hope someone can help narrow this down for me.

Thanks in advance.

Chris