No problem, thanks for the inquiry!
I would suspect the Start Cap is defective as well, you can actually find out how to test it if you're not sure it's defective by looking at the attached document.

As for finding the correct Start Cap for your model, it's discontinued of course so Sanborn may not sell an OEM replacement.

The best way to go would be to find the microfarad rating which would be denoted as "uF" or "mfd" on the capacitor itself. You would also need the "VAC" voltage rating on the capacitor.

You can take a look at the various generic Capacitors we have on our site by clicking on the below link if you would like:
www.mastertoolrepair.com/electric-motors...itors-c-102_335.html

- MTR

Understood, well you may want to reach a technical rep with Sanborn at 888-895-4549 so they can give you some of these specific technical details. Unfortunately that information isn't something I have available to me.
As long as the Check Valve holds air in the tank and isn't back-feeding through the line and putting pressure on the head, I would imagine it's an electrical problem, most likely a capacitor.
Thanks, hope this helps.
- MTR