Your front brakes are supposed to do most of the work.
Is it possible that the proportioning valve is not working correctly or stopped working, so that your rear brakes are now carrying the load?
Are your front brakes working at all? Maybe there is something in the line, or they are not adjusted properly, again forcing the rears to work harder.
Were the brake booster and master cylinder installed during the restoration 3000 miles ago? If the actuation rod that goes into the back of the master cylinder is not the correct one, or is not adjusted properly, it could cause the problem.