Your air bubbles are most likley comig from a bad pump,the air/foaming makes the oil expand and come out of your vent, this is not a common problem you just have not found the source of the problem yet.You should not have any foaming in the system
Make sure you have the engine number to get the correct hydraulic pump
Tommy
Affordable Tractor Sales
“Your Jinma Parts Superstore”
http://www.affordabletractorsalesco.com