overfilling the case will definitely strain a std breather, especially the old type with the silver band. even once you refill it to the correct level you may still have a leak, they often leak from the crush washer between the breather and case or pool around the top of the breather from a leak where the silver alloy ring joins the plastic top. it's amazing too where fast moving air will push fluids around the back of the rear cyl. i too doubt it's a base gasket..........but then i am wrong......often.