Oil coming out of tank vent pipe on Harley 45" flathead

[Buster]

A friend has a newly rebuilt stock 45" flathead. When he starts it oil comes out of vent tube in oil tank. I realize that this tube only equalizes pressure between oil tank and crankcase, so why would oil be coming out of tube into tank?, The motor only has 15 miles on it with fresh bore, pistons and rings. Bike is out of town so I cannot take picture of oil flow from vent pipe. Thanks!

[duffeycycles]

Are you sure it is vent & not return

[Buster]

Yes, it is the pipe opening at top next to cap. He has been filling oil to slightly above top line on dip stick (to between the two L's were it says "Refill"). He is lowering oil level to top line on dip stick, but don't think that will make difference.

[Rubone]

Yes, it is the pipe opening at top next to cap. He has been filling oil to slightly above top line on dip stick (to between the two L's were it says "Refill"). He is lowering oil level to top line on dip stick, but don't think that will make difference.

That is the return line.

[RichO]

I believe there's two tubes up there. One return and one vent just like on a big twin oil tank and supply on the bottom. one should be pumping oil back in.