If the hydro relief valves (2 of them on top of the hydro pump) are original, then they are manual and will have a small button on top. The lever is supposed to press a thin metal tap against these buttons to release the valve. The thin metal part is prone to breaking off. I wouldn't try to move it unless the valves are leaking and the pressure lets off after a day or so. Otherwise, you'll have to pull the fender pan and then you can reach inside the frame and manually depress the buttons.
You already know this now, but check your engine oil every time you use your tractors.
