Thank you for the post!

I would check a few different things out.

You said you looked at the Check Valve, just make sure it isn't clogged with debris.

Make sure the Belt is not too tight or loose. When you press down at the center point of the Belt there should be no more than 1/2" deflection. A Belt that is too tight or loose can cause this problem.

Have your Capacitors tested on your Motor. You have two Capacitors, one is a Start cap and the other is a run cap. They may be losing "juice" so as to not provide enough energy to keep the motor turning the pump. I would have these tested by an electrician at a motor repair shop or electric supply store. You need to make sure the Motor isn't drawing too much current, so putting an amp meter on the motor or having an electrician check it out is recommended.

Lastly, you could be looking at sticking Reed Valves in the Pump, but I would check the above potential problems first.

Let me know what you find out with the above (potential) issues and we'll go from there.

- MTR