O.K. Serial number 16800156 falls in with all the regular Big MO 500 serial numbers.
My Big MO 500 parts book (regular industrial, not reversed) shows a separate power steering pump mounted in the side cover of the engine. It should have a round top with a cover with a bolt through the top.
I guess you already removed the top from the pump and checked the fluid level.
If the fluid is getting away it could be working past the seal in the pump into the engine crankcase, or leaking past the piston seals in the front-on a reversed tractor it would be the rear-bolster. If its getting in the bolster it will eventually run out where the steering shaft enters the power steering valve or it will run out the vent under the radiator.
Your steering valve where the steering shaft enters the rear bolster is a Ross valve. You should have 650-750 psi going into the valve when the wheels are turning. (This figure comes from an I/T manual and is really for agricultural tractors with a pump at the rear of the tractor with a priority valve. But it should be about that much pressure or you won't have much steering assist.)
What I mean is the pressure from the power steering pump should be 650-750 psi when you are turning the steering wheel and the power assist is helping turn the tractor's wheels. Install a guage in a tee in the pressure line from the pump to test this. I think it will be the bottom fitting on the pump, but to be sure, the pressure line from the pump connects to the power steering valve where it says "In" or some such on the bottom of the valve.
My book says the pump should be a Vickers VT16-100-40-120-30-10S in case you need to come up with a seal kit for it.
FYI: According to my serial numbers, your Big MO was third from the last to get the 4 star sheetmetal. Later Big MOs have a flat hood and a heavy cast iron grille around the radiator.