Yeah, that's the same as my GC 1725, which is just a newer version of your ...
I'm not sure of the official name for the valve, but move the handle forward the 3pt drops until either you let go and it returns to center, or it hits bottom, it will then stay in that position until it's raised, by moving the lever backwards from the center position it raises it ... The rate of drop can be controlled by the knob by the floorboard, which can also lock the 3pt from creeping down (but not up)
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It doesn't have "position control" like my Kubota ... On that, once I know what height I want the 3pt at, I just move it to that number, the lever stays in that position until I move it up or down, say I was rototilling, and knew that #3 was good, at the end of the row I'd lift it by pulling it back past #8, then once I had turned around and was ready to till again, I'd drop it to #3 again and it would be in the same position as before.
With these little GC's it's more of a guess and eyeball it type of thing!