Thanks for the post!

Does the unit pump ANY air at all? Or just very little?

Either way, if you get no compression or very low compression, the problem most likely lies inside the pump itself, most likely you have damaged Reed Valves in the pump and/or a damaged Piston (Compression) Ring. The Piston Ring is a Telfon-coated ring around the piston and if it has any nicks, tears or folds along the outer perimeter of the ring then you won't get a good seal inside the cylinder, so that would be the problem.

The Reed Valves may also be broken or rusted onto the valve plate, so make sure to look at the valves. However, most likely the Piston Ring needs to be replaced.

I've attached the breakdown diagram of this pump here for your reference.

I doubt the start capacitor is the problem. It certainly could be, but you'll know if your start cap is defective if you turn the unit on and it just hums, and doesn't start. Does the unit start and stay running? If so, the start cap is okay.

- MTR