Below are a collection of fixes and ideas for repairing the check ball seat from the XLForum.

See also this Youtube video on the check ball and wet sumping by XLFORUM member bustert ¹⁾

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The pump already has an oil seal in the body of the pump.
This seal stops oil from leaking past the shaft on the scavenge side and into the feed side gears.
What HD didn't do was use an oil seal above the feed gears to stop oil leaking past the shaft into the pump tower.
So the shaft itself just runs against the cast iron pump tower until it wears enough clearance for itself to not interfere with the oils passage.
Once that happens the pump will leak.
With the bike leaned on it's kick stand, the oil leaks into the sump and then overflows into the primary case via the oil transfer valve in the left case half.
This really is a design flaw and should have never been allowed to persist.

This solution is to install a seal in the pump tower.
The biggest problem here is the area that should hold the seal is only about 3mm thick.
And the answer to that problem is to machine up a “top hat” to be pressed into the pump tower to hold the seal.
The seal used is the same seal used in the pump body (26227-58), about $4 worth.

The machining was farmed out to a local shop. It took them 1/2 hour to make two top hats, machine the towers and press in the top hats.
They also made two tools for the installation. Both tools are very simple for anyone with a lathe to make.
The first is a backing tool that stops the pump tower face from distorting (bending inwards) when the top hat is pressed into place.
The second is a seal installer. However, it can be dome without the seal installer.
After the top hat is pressed into place, the pump tower face is faced off.

So, $55 for the machine work and $8 for two seals.
Here is the sketch of the top hat. The measurements are metric but can asily be converted to inches.

Here's the machines tower with the hat already pressed in.
It should be clear to your machinist what is required from this. The actual line between the pump itself and the top hat is hard to see but it is there.