I would remove both the tank and the filter housing, wash them out in a parts cleaning tank and air blow dry. I would inspect the reservoir interior with a mirror and flashlight to make sure it is clean. The hoses also need to be addressed. Metal particles have a way of accumulating where hoses slip onto fittings and threaded fittings at the end of the threads.
This type of hydraulic drive system once filled with oil and operational basically circulates oil from the pump to its motor. The pump has the ability to vary and reverse the flow of pressurized oil to the wheel motors. The reservoir keeps a head of oil above the pump inlets and provides a place for return oil from the pumps thru the filter. The amount of recirculation of oil from the reservoir varys during machine operation.
At this point in time you now have a better understanding of why the Scag servicing dealer quoted to you what they did back in the beginning of your hydraulic drive problems.
