Do the rear brakes work at all? A single line master cylinder pressurizes all the brakes equally- - - -unless there's some kind of restriction like the rear flex line being collapsed that prevents pressure from getting to the rear axle, or a pinched steel line to the rear.
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!