Started teaching HPDs back in the mid 60"s. You can drive the unit on the bench with a half inch electric drill. Take a radiator hose of the correct size, slip it over the input shaft and the outside of the drill chuck, clamp it on with radiator hose clamps.
Direct drive clutch pressure should be between 140 to 190 PSI. Tee in a pressure gauge in the hose that goes from housing to the collector ring tube that comes to the outside of the housing. Lube pressure should be between 20 to 60 PSI. Take a 1/8" pipe union and weld it to the top center of the filter. After welding, drill a small hole through the filter, you are now on the clean side of the lube system. Since you are now probably running the unit at about 1/2 rated speed of engine, you will probably see pressure at the low end of the spec. Remember the clutch is a closed center system, thus once the clutch fills ideally it should not require more fluid. Watch for pressure change between hydropower drive and direct drive, should only drop a couple of PSI. If pressure drops really low you have a leak in the clutch circuit or clutch piston rings.
We always recommended draing the type A oil and putting in a mixture of diesel fuel and one quart of oil since this will duplicate the viscosity of the type when it is at operating temp.
You should see the change in sprocket speed change when shifting, you may have to put some resistance against the sprocket to see speed change.