How cold is it and what kind of oil is in the hydraulic system? PTO engagement shouldn't matter, as the hydraulic pump is driven right off the engine gearcase so if the engine is running, the 3PH should work. IF it is below freezing and there is any moisture in the oil, it can have ice crystals blocking flow to the pump. If it has UTF instead of hydraulic oil in the hydraulics, it could be too cold to flow properly. IF there are any leaks between reservoir and pump inlet, thick oil will make it more likley for the pump to suck air into the pipe rather than pull fluid along/thru the pipe. A lot of the Jinma's have a pipe union at the top front of the reservoir on the suction pipe. This is a prime place for an air leak.
There is a valve handle down in front of the seat, kind of like a water faucet. This controlls the speed at which a heavy impliment falls when lowered. If it is screwed all the way in, it will stop the 3ph from working by blocking flow to-from the 3ph cylinder. But if this is the case, it will also cause the engine to bog down as the fluid builds pressure when it cannot reach the cylinder and the relief valve opens in the 3ph control valve when you try and raise the lift...
A quick check for some of this is to get the tractor into a garage and turn a heater on it to get it up to a warmer temp. If it works when warm, it is probably a water/suction/too thick fluid problem...
