Saw this earlier and started to respond, but Mrs. waste wants a "clean house for a clean year", so I had to do some housekeeping

As we nor you know how long the pump ran without a prime, I suspect that, as Al said, some cavitation could have occured while the pump ran w/o a prime (what caused it in the first place, I'm not sure- sounds like it's the return valve you're talking about, and that wouldn't cause a suction side air leak). If the pump did indeed cavitate for a prolonged period, some of the seals (pump housing lid gasket, seal assembly or housing body gasket) may be leaking, though only the lid gasket will allow air into the pump to kill the prime, however, the pump inlet fitting may have warped and is now drawing air

You can tell if you have a suction side leak by looking to see if you have air bubbles in the pump housing. If you do, you can try to isolate the problem by slowly pouring water over any suspect area and watching to see if the bubbles go away.