The limited experience I have with hydraulic control valves revolves around a Meyer 2 spool snow plow valve from the late 60's and the 2 spool valve on my 1955 IH tractor. The snow plow valve has/had a check valve that would hold the plow in the up position until the manual valve was positioned to lower it, internally it would release the check to let the blade down. I would expect that your hoist valve would be similar, the check would most likely be internal within the valve body. You may have already found the culprit if the valve was unable to fully extend to "HOLD" position.
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
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