I'm trying to get to the air filter, but the air filter box won't open. All the screws were rusted pretty tight: some WD-40 and some elbow grease took care of most of them but there's one persistent little guy that won't go away. The screw is probably damaged, because while it will turn, it won't come up. I tried pressing up against the bottom end that protrudes out of the edge with a second screwdriver, but no luck. Any tips?