1988 Jeep Cherokee No Rear Brakes

I have recently had all the brakes done on my 1988 Jeep Cherokee. We had replaced the front rotors and shoes. Then noticed that the rear brakes were not engaging. We replaced the rear wheel cyclinders, bled the lines and still no back brakes. We then replaced the master cylinder and rebled. Still no rear brakes. Just yesterday, we disconnected the rear brake line to double check we have good fluid flow to the back and got a healthy squirt of fluid when stepping on the brake pedal. However, there is still hardly any brake action on the back brakes. We plugged both master cylinder ports to double check the master cylinder and it checks good (super solid pedal). Plugged the rear brake port with front brakes attached, front brakes work fine. Plugged front brake port with rear brakes attached and the pedal is very solid, but no rears brakes are engaging. We have rebled numerous times. What else could be the problem?