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

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.

Asked by for the 1995 GMC 1500 Pickup
maybe the abs module is broken