If you used a master cyl. for that model truck, it will have a check valve built into the rear of it. That master cyl. was never used for disc brakes & a check valve is part of it. To check the function of the check valve, pump up the brakes & release the pedal. Wait about 30 seconds & crack a bleeder. A small amount of fluid should come out like it is under light pressure. Then it will stop coming out or just drip a little.
My opinion is that is not your problem.
George
