Sounds like maybe you have a leak in the suction line. The water boil point lowers under vacuum so potentially sounds like there's a leak somewhere on the down side of or below the check valve.

Re-prime the pump, run it for a couple minutes with one of the irrigations zones turn on. Shut off the pump. Then stick your ear again the piping on the suction side in several locations. You'll probably hear bubbling as you're losing prime. Your situation sounds like a very significant leak, hopefully at an above ground connection. Is there a coupler on the suction side?

Mikie,
Thanks for your input,
I replaced the check-valve and that did not solve the problem.
How can I determine if there is a leak on the suction side?
Thanks, Erik

Erik, I wasn't kidding with the recommendation I posted. I solved my own leak problem this weekend using by carefully listening. In my case the coupler wasn't sealing correctly.
Steve