I would suspect a stuck check valve but it would have to be "really" stuck for the pump not to be able to lift the valve off it's seat. I would bet that there is a spring loaded pressure relief bypass valve in the circuit, you might want to investigate that avenue.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy