It could still have water in it or it's getting some with the air, but if you've cleared it out, no sign of water is coming out of the exhaust and it's still stalling it could be done or it could have a ruptured diaphragm. That will allow air through the pump instead of liquid pressure. I've taken them out of another old pump and gotten some life out of it in an emergency, but again I usually just replace it and don't worry about fixing it. There are three kits to do a rebuild, any two of them cost more than a new pump, and I usually can't see anything wrong with them when I take them apart so I don't know what to replace to assure they'll even work again, they're a pain to work on, and my time isn't worth possibly saving a few bucks on these so I toss them.