1995 GMC 1500 Pickup Question: What causes pressure failure to the back brakes? See details.

ABS system will not let the fluid to the back brakes. Front brakes are fine. There is fluid pressure from the master cylinder to the ABS module but the pressure cannot get through the module and there is no pressure going to the back brakes. The problem is continuous and I do not have a diagnostic code. Thanks for any help you can provide.