Without knowing or seeing your setup, it's not possible to tell if you have a secondary hydraulic pump for your loader. Some of the utility versions had a second pump mounted at the front of the tractor.
That being said, the DOM valve is supplied with oil from the main pump via the valve assemblies under the fuel tank. If you have remote hydraulic couplers (or the loader uses these valves) , the DOM supply port will be on the top of one or two valve assemblies. Follow the supply hose from the DOM valve forward along the right side of the torque tube to find that connection.
If in your shoes, I would locate a hand-held/portable pressure gage and find a way to temporally plumb it into that supply port to check for pressure. That will indicate if your problem is in the pump of valve assemblies under the fuel tank, or if your problem lies in the DOM value.
If I remember correctly you have indicted that you are not seeing a drop in your hydraulic oil level, and a corresponding rise in fluid level in the transmission. If that is true, it potentially rules out the rock shaft piston seal. That would defiantly be the case on the early tractors, but if I remember correctly, Case devised a channel on the late models to plumb hydraulic fluid that got passed that seal back into the torque tube/hydraulic reservoir. Since yours is a 1968 model, it leaves that possibility open.
I would first ensure the hydraulic filter has been replaced and properly re-installed. Then I would check the hydraulic pressure in the line that feeds the DOM valve. Then plumb the pressure gauge into the connection between the DOM valve and the rockshaft piston. If that checks out, I would then pull top plate off of the transmission and inspect/replace the rockshaft piston seal and wiper.
The system is very simple. The issue can only be in pump/valve assembly system under the fuel tank, the DOM valve or the rockshaft piston.
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