“It has to be something about the valve
Reviewed the Hydraulic Equipment SERVICE MANUAL John Deere Waterloo Works SM-2011 (Dec-52).
Take a look at the WILL NOT STAY IN RAISED POSITION troubleshooting list below.
Note the second item Check valves leaking. Reseat.
Take a look at the Orientation Diagram below.
Note the CHECK VALVES, TO CHECK VALVES, and OIL LINE COUPLINGS.
Take a look at the TESTING CHECK VALVES FOR LEAK topic below.
Hope this helps.