sta-rite pool pump not primming

[habudab]

my pump is not priming properly. I noticed it started when the union was leaking. so i replace the union and it is tight and sealed at this point. I took the pump apart and replaced all the seals and the diffuser, because it did not look great. the impeller looked fine and spun freely, i didnt see any cracks or anything that looked bad with it. i put it all back together and filled area with water and when i turn the pump on it sucks the water from the bucket, but thats it- it wont pull much at all from pipe. so i checked for clogs etc etc- i saw nothing, so i decide to put a vacuum hose in the suction pipe and was able to suck water from pipe and fill up bucket. but when i close everything off and seal it up, it wont prime, but i can hear the motor running and it suck up some water, but wont prime..... any ideas???? pump getting weak, doesnt sound like it is to me???? thanks in advance for help!

[Denniswiseman]

Sometimes theres a non return valve flap inside the pump broken
Fit a NRV on the suction side of the pump, fill the pump basket with water and start
You may have to do several times