Did a front disc conversion and am trying to bleed my brakes. After a couple quarts of brake fluid still no rear brakes. Have good flow from calipers while bleeding, and flow from the back brakes not a strong stream. Put car in the air and started engine let out the clutch and rear wheels still spin with no resistance with pedal smashed to the floor. Front calipers lock up solid but nothing on the rear. Rear brakes are adjusted as tight a possible, was told it could be a bad master cylinder, cup torn. Tested the proportioning valve and no light was on, therefore it must be centered. Took out the master cylinder and am going to get another. Hpe you guys can help with this pia problem running out of ideas.