If the problem is isolated to only the rear wheels, check or replace the flex line between the frame and the rear end housing. Something is holding pressure in the rear wheel cylinders, and the flex line can collapse on the inside and restrict the fluid flow back to the master cylinder.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!