The control valve should have a check valve built within, that would be my first thought of not holding pressure, the oil could be bypassing internally within the cylinders but the control check or worn valve spool would be more common.

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