Don,
My L1501 was doing the same thing. Underneath the seat on the left side of the housing, there is a vent tube that lets the case breathe. It is a small crook shaped piece, about 3" or so long. Dirt daubers(Mud daubers) have a nasty habit of making themselves a cozy lil' house there, in turn, plugging up that vent tube so the case can't breathe. Take a small piece of wire or something similar and carefully try to dislodge the dirt or debris, if there is any in there. The tube can be removed for cleaning if need be, but I didn't have to. Just try not to push anything further up in the tube. This fixed my leak problem. Hope this helps. Bill