No worries, thanks for the post!

This can be caused by a couple different things. It's leaking at the Centrifugal Unloader on the endbell of the pump, yes? Just want to confirm.

1. The Centrifugal Unloader Valve needs to be replaced. If so, we have this available on our site for purchase at this link: www.mastertoolrepair.com/unloader-valve-76281-r3076-p-5704.html

2. The Valve Discs & Springs in the pump need to be replaced. This is a pretty typical problem, we have Valves available on our site here: www.mastertoolrepair.com/air-compressor-...arts-462-p-5416.html

Also, does the valve leak constantly while the unit is running or when it shuts off? Or both?

- MTR

Ah, that's telling.

In that case, the Unloader Valve is either defective or the one-way Check Valve in the tank inlet (which your main supply tube from the pump head routes to) is not sealing.

The Check Valve problem is the most common problem with any air compressor.

Here's a simple test you can do to confirm:

1)Let your unit pump up. 2)Turn it off. 3)Find the supply line from the pump to the tank. 4)Remove the supply line from the pump head. 5)If you hear air coming from the tank you have a defective check valve.

NOTE: You could also remove the small 1/4" bleeder valve that routes to the pressure switch, as that routes to the check valve as well. Either way, if you get back pressure from the check valve, then it's not sealing and you either need to give it a good cleaning or replace it.

You can click on the below link to view a technical document as well as a short video:
www.mastertoolrepair.com/how-to-test-check-valve-p-22524.html

- MTR

That is a very small compressor, of course, but the concept is exactly the same.

It's easiest to simply remove the main supply line (which is at least 3/4" I imagine) from the outlet of the pump to the tank. Where it enters the tank is the Check Valve. You'll notice right away if air is leaking back or not, as soon as you begin to loosen the compression nut.

- MTR