i've done quite a few of those M-40's with bad/leaking (internally) position control valves. On most of the cabin models you have to lift the cab a little bit to get the top cover off to get access to the PCV. Then you can take the valve off, take it apart, free up the sticking valve(s) and reassemble. Make sure you have a service manual handy as there are a couple of adjustments that need to be made.
but before deeming it bad, you really should test the hydraulic pump-the correct way. Pressure AND flow. Need to make sure the pump is not damaged and the rest of the system feeding the pump isn't restricting flow.
the valves can get sticky in the pcv which causes an internal leak, which reduces pressure available to the rockshaft cylinders Raising RPM increases hydraulic fluid flow enough to make a little more pressure. Remember, pressure is a sum of restrictions in the system. Sometimes you can get them apart to repair, other times not. On the majority of the ones I did, I found that they were using tractor supply company "Kubota compatible" hydraulic fluid. I can't blame them, it's cheaper, but there was question as to whether the hydraulic fluid might have contributed to the valve sticking. I couldn't answer that question other than I've seen a few of them with the same issue and they all used cheap fluid.