4 valve is a 4th potential problem,,, the valve itself, servo controlling it, and programming to operate it.
Prv, pressur relief valve, is the same thing that the 4th valve is doing, just without all that extra stuff, a simple spring and bb (so to speak)
For what I have been able to conclude about others using a valve to alter flow is for a possibility of non labor on the pump itself. With the servo controlled valve the oil is under no restraint (pressur) when no other valves are in use,(no stick com,and) thus releasing the flow unrestricted to circulate through the system. And not causing the motor to labor anymore to build pressure. Sounds good but just more things to go bad in my opinion. And for the Jung style pumps and planetary reduction on pump gear shaft it's just not really gaining much. Run time? Well if I can run for over an hour it may get me to add a couple min more in reality. So not much advantage. Now on the gear mesh pump the 4200 has, reliving pressur may help but again any real significance? Probably not. Prv, set it and forget it!! But one thing is to tie in an actual dial gauge to see what pressur you are actually running. It can be removed after set up or just stay if space provided.
As far as motor on /off. The motor isn't too bad as far as heat from constant start stop but the esc will not like that. Fact of our type of speed controllers is that are coolest at full throttle or at max signal input. By stopping/starting running half speed that is what esc's don't like and generate more heat when used that way. Now for efficiency, motors suck down electricity during initial start up. So if you wanted to stop /start pump with stick input , the best bet would to keep the pump running at say 30% then ramp up to your max rpm for stick input. A lot of programming but easily doable. Originally I had that with my ex but changing radios I just said ?!$$& it, and put the pump on a dial pot with switch to turn it off on. That way if I'm in sand, not much pump speed needed, harder soil I can turn it up. (Longer run time, but not really a pressur gain as I have the prv set) I think I have the converted 4200 running around 350-375 psi, that a bit higher than normal but I made it heavier than normal too
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And some way to achieve the realism of hydraulic movements it to use your end point travel on your servos settings. That will allow the actual valve to only open so far thus restricting the flow of oil resulting in slower ram extension/retraction. But still allow whatever pressure you have set.
And a bonus is you can have an extra valve for say a thumb if you eliminate that way of controlling flow.