M Two Way Hydraulic Leak

Given that you probably can't find the original-style cap with sealable vent any more, a plain old 3/4" black iron pipe cap will work. You can switch caps back and forth as necessary for one- and two-way operation.

To add enough to that fill pipe so it won't blow out would be tremendously inconvenient and it would make the tractor look stupid. The pipe would have to extend around and above the gas tank.

Mount it under the gas tank in a convenient location with the vent on top. Make sure you can get to it easily to check the level and fill.

Disconnect the line from the OUT on your two-way valve to the fill pipe on the belly pump. Remove all extra fittings from the fill pipe.

Run a line from the lowest bung on the tank directly into the fill pipe of the belly pump. No vents, no tees in this line. The venting is done by the auxiliary reservoir.

Connect the OUT line from your two-way valve into another low bung on the external tank.

Now the oil flows from the external tank, into the belly pump reservoir, through the pump, into the valve, and back into the external tank.

The best way to return the fluid is at or near the bottom of a reservoir. You can use the drain fitting but it really should be 3/4" NPT instead of 1/2" NPT.

Oil returning above the fluid level in the reservoir can cause foaming of the oil.