I keep losing prime

This problem just cropped up over night, been working fine all year. it is a self priming pump, and has worked perfectly up to this point. Now it takes about 30 seconds to prime and only keeps it for about 1 second then a big burp of air seems to come from somewhere and the process starts over. I have checked all the plugs and O ring, seems ok. doesnt seem to be any blockage, but i dont really know how to confirm that
 
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