I concur with the leak somewhere. I had this happen too, and finally found mine -- the whale gusher foot pump was leaking air into the line. The water tank fed "T," one branch went to the water pump, the other to the foot pump. The water pump developed enough suction that it sucked air through the foot pump somehow. I just stuck a plug into the line for now.
Which reminds me, I need to order a new foot pump! The thing looks like original equipment and the plastic looks yellowed and brittle so I don't want to rebuild it. Defender has them for 100 bucks. It isn't often you can get a piece of equipment you need for a tenth of a boat buck.