Usually they get sticky and gummed up from sitting. You can take them apart, clean , oil and reassemble and they will work. Look for worn out O-rings, I think the trigger may have an o-ring, if it does not seal, it may run constantly. Just look for worn parts and go from there. It is like rebuilding anything else, usually the rebuild kit doesn't have what you need, so I take things apart, clean inspect, then make a list and order parts, then reassemble.