CAT 769D dump truck

How can you release the parking brake if you suspect the parking brake pump that is mounted on the torque converter has failed. Pressure is needed to release the parking brake because they are spring loaded in the rear hubs. Tranny speed sensor checks ok so does parking brake release valve. When scanner hooked up no trans rpm showing in any gear and no movement forward or reverse. Removed rear drive shaft then I get rpm and Tranny
up shifts. This is why I believe even though parking brake shows to be released its not.

CAT 769D dump truck