Pool and Spa
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I have seen this a lot before and in most case it is a stuck weir in the skimmer mouth. There are white flaps that bob with the water and if one gets stuck it blocks the water going into the skimmer and then the pump pulls all the water out of the skimmer until it pulls air then the pump drops out the skimmer fills back up and the cycle repeats. Look into the front mouth of the skimmers and push the weirs down with your hand then turn pool pump on and see how it works.
OK so from there we need to look at all areas around the pump for air leak, check pump lid o-ring be sure it is not missing or pinched, get some lube for o-rings an apply it to lid o-ring, check pump lid for any signs of cracking. I just did a job the other day where the pump was doing the same thing and I opend the pump housing found a stick and leaves in the impeller clear that out but I did find that the union at the front of the pump where water enters was not tightened all the way and the o-ring needed lube, once I did this the pump pulled water nicely and ran fine. If you have no union in front of pump you still will have a male adapter threaded into pump housing this could be warped or cracked and the pump draws in air.
ok no problem