Thanks for the post Smitty.

Knowing the model# of the unit, I can say with pretty good certainty that the problem is most likely the Reed Valves in the pump head.
You can see a schematic and parts list of this unit on our site by clicking on the below link, btw:

www.mastertoolrepair.com/portable-oil-fr...6020wk-p-759564.html

The Valve Plate Assy is how it is sold and includes the valve plates with reed valves pre-installed. You would also need to order the "D30139" Gasket Kit when replacing the valve plate, which also includes the Tube Seal that seals the transfer tube inside the head; this is also a very common part to leak as it eventually deforms and destroys the seal at the exhaust in the pump head, so it's included.

Most likely you'll find that one of the reed valves in the pump head is either rusted or damaged and/or a damaged Gasket, therefore not allowing the unit to pump to its maximum PSI.

Also inspect the Piston (Compression) Ring; if it's torn or chipped along the outer edge this can certainly cause the problem you're experiencing.

Hope this helps!

- MTR

You can, sure. Or, you can simply search "Y6020-WK" on our website and click on the first result which will bring you to the schematic for this unit.

Feel free to post here once you tear it apart and we'll go from there.

- MTR