Thank you for the post odis,

You guessed right, the problem is the reed valves. Most likely the exhaust valves, specifically, are not opening as they should to exhaust the compressed air so the air is escaping from the inlet filter (which is the tell tale sign of defective valves). This is also the reason you're getting oil in the head.
The valves are only available in the valve plate assembly, which is part# 043-0207 and includes the valves pre-installed on the valve plate and both gaskets.

You can view it on our site by clicking on the below link:

mastertoolrepair.com/portable-single-sta...221495-p-759576.html

Hope this helps!

I doubt the check valve would cause the problem you're experiencing, no. If the check valve were defective, you would only get back pressure (and hence a leak from the inlet filter) after the units shuts off, not while it's running. It's probably best to replace the valve plate assembly as it comes with new gaskets as well.
You can attempt to replace the gaskets only, but I'm betting the valves in that right head are not opening and closing as they should. A good cleaning will help greatly, as well.

- MTR

Odis,

This is strange...just to confirm, you have suction from the left cylinder's intake filter, and it's exhausting air form the right cylinder's air filter?

- MTR