Sounds to me like your prop. valve is holding pressure on them after you let off the brakes. The prop. valve for disc/drum acts like the residual pressure valve as well for your rears, so it could just be keeping the fluid held after you release.
Mike can fix you up with one for cheap if that's the problem. (He's a board vendor).
www.bumpandgrindgarage.com
Might want to listen to another person though before going with just my advice.
DLB