Here's the thing about Jinma hydraulics: they shouldn't work, but they do. It's like the old saw that aerodynamically a bumblebee shouldn't fly, but it does.
Jinmas have what are called "open center" hydraulic system. In an open center system, a valve is open -- allowing fluid to flow freely through it -- when it is centered -- neither up nor down, ie not doing anything. Open center systems have a constant volume pump, it moves the same volume of fluid with every revolution of the pump, and fluid is constantly flowing. When no work is being done the fluid is at zero pressure. When a valve is activated to do some work the resistance of the circuit increases, and the pump increases the pressure to keep the fluid volume constant. The valve directs fluid into one side of a cylinder at pressure, which moves the piston, and an equal volume of fluid, not at pressure, comes out the other side. So the flow in the circuit remains constant. Every valve has to have a pressure relief that allows flow once pressure exceeds the rating of the relief valve. If flow is interrupted and there is not a relief the pump will increase the pressure until something gives.
In an open center system all of the devices are arranged in a series, and fluid flows through all of the valves upstream of the one in use at full pressure. Every valve should have two outlets: a power beyond that provides fluid at pressure to downstream valves, and a return to tank that bypasses the downstream valves when the valve is in use. On the last valve in the series there only needs to be one outlet, a return to tank.
Jinma plumbs their hydraulics differently. The loader valve does not have a power beyond. It has a single outlet, as if it were the last valve in the series. But it isn't, the 3-point hitch follows it, and the outlet of the loader valve feeds the 3PH. The reason that power beyond is necessary is to protect the system from high pressure. The pressure relief valve in each valve measures the pressure difference between the input and the outlet. If you have two valves in series the total pressure at the pump is going to be the sum of the pressure over each valve. So if you have a pump that is rated at 2000 PSI, and the FEL and the 3PH each have releases that are set at 2000 PSI, if you use both at the same time and you have Jinma plumbing you would see 4000 PSI at the pump. And your right foot would probably be covered in hot oil in a few seconds.
Now somehow, Jinma gets away with this. I don't know if it's just rare for people to use both the FEL and the 3PH at the same time, or if there is something else. Jinma uses standard FEL valves so it's not something special about the valve. I do know that on my tractor if I use the steering and the FEL valve at the same time I get unpredictable results.
What gets tricky is when you want to add remotes. The proper way to add remotes would be to have the power beyond of the FEL valve feed the remotes, the power beyond of the remotes feed the 3PH, and the return to tank of the remotes tee in with the return to tank of the FEL. But the FEL doesn't have a return to tank, and it doesn't have a power beyond! So how to proceed? I can think of four ways:
1. Replumb the FEL valve "properly" with a power beyond and return to tank, and then plumb the remotes normally.
2. Leave the FEL alone, and put the remotes upstream of the FEL plumbed normally, with power beyond feeding the FEL and its own return to tank.
3. Put the remotes valve in series with the FEL and plumbed the same way as the FEL.
4. Replace the FEL valve with a 4-spool valve plumbed the same way as the existing FEL valve.
Options 1 and 2 are complicated by the fact that some Jinmas (like mine) don't have any easy place to put in a return to tank. You'd have to drill a hole in the tank and tap it with pipe threads.
As noted above, the risk of not having a power beyond and return to tank on any valve upstream of the 3PH is that excessive system pressure could destroy the pump. That risk could be mitigated by having a system pressure relief valve on the pump outlet that opens to the tank if there is excessive pressure. Many people believe that Jinma should have done this to begin with, and I know some members here have made this modification. However, to do this you need a way of tapping into the tank.
