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Welcome to JUST ANSWER heavy equipment. With years of experience, I look forward to doing my best to help. Do you have the hydraulic pump mounted on the rear of the engine, over the transmission or the large pump mounted on the right side of the transmission behind the step? Most likely the pump is weak and looses its pressure when hot. The tractor should only need the Ford 134 hytrans fluid and would not use 80w90 gear oil. With the gear oil, pressures should be even greater, but with the heavier weight oil it doesn't work, making me think the pump needs repair or replacement.
right side behind step.
Pump needs to be examined and most likely replaced. It has pipe that goes to the flow control valve that has to be slipped up into the flow control to allow pump to be removed.
First you might want to check # 29, which is the relief valve. In the bottom picture , the # 41 is the tube that has to be unclipped, to allow it to be pushed into the flow control to allow pump to be removed.