I haven't bought any hyd hitch parts from Case for 5 yrs but practically everything was avail at that time. The relief valve spring is Case p/n V14857, cross-ref p/n A30709.
Go to the link and click on page G7 D2, the relief valve is under that plug between the pressure control valve and the release valve plungers. See the valve parts and the access procedure in the pages following. It is a simple spring loaded ball with a retainer. Not much to go wrong there. The spring can be jury-rigged shimmed, not recommended. Test pressure per page G7 C11 if shimming, design relief press is 900 -1000 psi.
Before you start tearing things apart, check these 2 things first as they affect lift travel & capacity:
1. Back to page G7 D2 & pages below, inbd the hyd lines see the "breather line". That is the 5/16" tubing that vents the rock shaft housing to the torque tube sump. The purpose is to prevent an air lock under the piston, it also conveys the inevitable oil and grease accumulation in the rock shaft housing to the torque tube. That tube may plug up with hard grease and impede piston travel, causing the relief valve to lift with the same symptoms as a defective spring. Take the tube off and operate the lift, some oil and grease pushed out is normal but after about 10 cycles it should be down to a slow drip. You should be able to feel air drawn in and blown out with piston travel. This is how you check for piston seal leakage also but that is for another day if you need it.
2. Now go to the bottom of the section and click on “Next 14” and then click on page G7 D13 and look at Fig D-39. The pressure control valve is on the right and release valve on the left. The lash between the adjusting screws and the valves plungers is 010”. Not critical but if excessive lash on the press control valve plunger the valve will not be fully closed and directing all the pump output pressure to piston. On the following pages are some tests, you can run the engine and operate the lift using a screwdriver at this stage, no problem, no oil bath.