coming out of the breather cap right?
when you're checking the oil levels are you doing it with the unit hot or cold?
it should be done with the oil at operating temp only.
If you are checking it when the oil is cold and adding it to get it to the top glass then when it gets hot it is probably expanding alot and making it overfull.

Try checking it the way i said, drain out some if it is overfull when hot till you see it at the lower hemisphere of the upper sight glass, not above that. Also clean out the breather cap since it has oil in it, use a fast drying cleaner like electric motor or contact cleaner so it will not damage the element inside it.
Then come back and tell is if there is any difference?
If it is still forcing fluid out of the breather cap when lowering then there is something wrong inside the tank.