Having never been around a D series, I have to ask....isn't the motor a gear motor & not a piston unit? I know the C series were gear motor, and the GT14 as well.
The D series uses a sundstrand piston to piston pump and motor very similar to the sundstrand 15 series. I am not very familiar with other WH products but some of the hydro tractors that are older than mine (1976) came with a Hydro Gear unit. The hydrogear is covered in the same manual as the sundstrand and it dosent seem to be as servicable as the sundstrand. I have read that the factory fix for a bad hydro gear unit was to update it to a sundstrand. On the d series the pump and motor are in a "divorced" system. The pump is located and coupled directly to the engine the hydro motor is bolted directly to the rearend (just like a case) with steel tubing and manifolds connecting the two together.
Here are a few pics of the motor and cylinder assy.