When I first had my KK, if not used regular it would drain the tank into the engine in about two weeks! At that time I wasn't aware of Harley's first theory on this so milled the oil pump body to take the later drive shaft oil-seal 26227-58. That fixed it! The oil must have been seeping from the tank (open to the feed gears), though the clearances between gears and body via drive shaft clearance (whacking great slot for roll-pin) into the scavenger gear compartment which is permanently open to the gear-case. Assembly is a little fiddly as the second roll-pin can't be fitted till the shaft it fully installed.
However, Harley's first solution in '55 was to do away with the over-pressure by-pass valve. They theorised that this, also being open to the oil tank, was leaking oil past into the gear-case. All I can say is that my '53 with seal installed doesn't leak-down anymore, so it can't have a leaky by-pass valve.
In the same way, if the check-valve doesn't seal properly, this too can provide a drain-down route. And I guess this is the problem with my '56. There's no by-pass valve, I've installed an oil-seal and it leaks down a lot in about a month. This I can live with as it won't be often it's not run at least every fortnight.
In short, the oil-pumps benefit from close inspection and perhaps careful lapping-in of the seats. I have a check-valve ball silver-soldered to a rod ready for if I ever take it down.
Having said all that, oil pressure without the by-pass valve is massive on cold start-up - about 85psi! Guess what, base gaskets weep till oil warms, then the pressure's down low - about 6psi at 40mph.
Talking of low pressure when hot, apart from the obvious factors, some aftermarket gaskets are too thick. The OEM ones are about 3.8 thou, aftermarket up to 7 thou (the one between the pump and engine case is correctly thicker). The gears are flush with the pump body, so gasket thickness is the running clearance - too much clearance equals low pressure.