Check your rear brake hose from the body to the axle and the proportioning valve. Either the rear hose has collapsed or the proportioning valve is not letting fluid to the rear of the car. Fluid is from the master cylinder for the rear brakes is probably stacking up solid on its way to the rear brakes somewhere, that would make effort very high on the pedal. I would start at the newly installed proportioning valve and work back to see where you can get fluid back to.
