I found this it might be helpful>.Your check of pulling the line off to the tank and seeing how much pressure is being generated is a good one. You have now pretty much isolated the compressor lack of pressure as being upstream of that point.

I would pull the intake filter too, and do the same check. Is it pulling air in strongly, or is it huffing air back out too? If huffing, that's a sign that the intake valve is pooched.

If it is pulling air in, and air is not being pumped out, yup, it could be a sign of cylinder piston ring wear, but then, you would be getting a lot of air out of the oil fill cap which is vented.

If that isn't happening, and you are getting strong suction at the intake, my money would be on an internal pump gasket blown which is allowing air to flow around inside the pump from high pressure to low, but not down into the tank.

When you open up the pump, you might as well have a new valve plate and gasket kit, as both are, in time, wear items inside the pump.