Hi Michael,

There could be many reasons for unbalanced rear brakes. If they are disc brakes then it could be simply a sticking calliper or air in the lines requiring them to be bled. If there are drum brakes at the back (most likely), then a brake shoe could be damaged or the cylinder itself is leaking on one side. We'd highly recommend you bring the car to a good mechanic to diagnose and fix the problem for you.

Best of luck.