You’re correct that the O-ring for the moveable piston is missing. It should be part # 350667R1. Without the O-ring there is no seal, so the check valve won’t hold pressure. The purpose of the check valve is to prevent the hitch from dropping due to pressure bleed down back in the control valve due to normal wear. It's essentially a safety valve.
I have a single acting check valve on my 340U and a double acting one on the 240U. I rebuilt both valves because the O-rings were badly deteriorated. Your photo looks just like mine did except the O-ring isn’t there. Apparently the earlier models used the single check valve.
The single check valve is supposed to prevent the raised hitch (up pressured)from dropping under load. The double check valve works in both up and down directions. In other words, what it adds is holding the hitch when it is used for down pressure, such as using the hitch to raise the rear wheels off of the ground.
Here’s the blowup of the double acting check valve. I'll see if I can find a schematic of the single valve hookup
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Today's Featured Article - Timing Your Magneto Ignition Tractor - by Chris Pratt. If you have done major engine work or restored your tractor, chances are you removed the magneto and spark plug wires and eventually reached the point where you had to put it all back together and make it run. On our first cosmetic restoration, not having a manual, we carefully marked the wires, taped the magneto in the position it came off, and were careful not to turn the engine over while we had these components off. We thought we could get by with this since the engine ran perfectly and would not need any internal work. After the cleanup and painting was done, we began reassembly and finally came to t
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