I searched the forums and found a lot about oil pumps leaking but no clear 'usual suspects'. Here's my situation:

Deet rebuilt the oil pump on an RT3 a while back and since then I took the bike on a 1000 mile ride in Colorado. Bike did great, I didn't notice any leaks while riding. Got home, washed the bike and made a few minor repairs (nothing dealing with the pump or oil tank/etc). Walked out to the shop a couple days later to see clean 2 stroke oil on the ground under the bike. Took the pump cover off and a fair amount of oil was collected there under the pump. Cleaned all that out, asked Deet about it and he said to check the gasket between the pump and case cover. I replaced that gasket with a new one (a fair amount of oil came pouring out when I pulled the pump off the sidecover). Cleaned it all out again. Didn't bleed the pump - just let it sit. Still leaking. I can't see where its leaking from. I used new copper washers on the banjo bolt on original installation (supplied by Deet). I don't want to keep bothering just Deet on this so I figured I'll bother several more of you with this post

Any ideas?

If it's been sitting and you cleaned it off well beforehand, lightly dust the pump area with flour or baby powder...  (I like baby powder because the bottle is conducive to "blowing dust" out of the holes in the lid with a simple squeeze).

If it wasn't cleaned well, clean it good, then dust the area with flour or baby powder.

Powder will stick to and show the trace of the faintest seepage. 

Just look closely at the plug in the end of the shaft then the seal behind the white plastic wheel. Hope it's none of those but gotta start somewhere. Also I guess they must have a seal or Oring on the shaft that moves out & in the other end. Not in anyway doubting repairs done but have to start somewhere. Another thought--remove the pump again then seal it off with something--wine bottle plastic cork etc, then you'll have the pump away from the motor so maybe easier to study around it, with oil line connected if course.
 

I rebuilt my oil pump several times, I couldn’t get it to stop leaking.  What I finally figured out was that the line into the pump had a cracked right at the pump, I cut a little off and it’s been great ever since, hopefully yours is something simple like mine was.