The pic of the pump sure does look like an air compressor. If you're going to try to use the air pressure system to flow water at a acceptable rate the system must be well sealed to hold at least 30 lbs psi, and the old air pump must be able to produce 30+psi. You have'nt mentioned a pressure gauge in the system to determin the pressure in the tank. If the "air nipple" on the fill head is like the ones I've seen it accepts a common tire pressure gauge [the best old air systems had a pressure gauge built into the fill cap]. Try this. Add a few gals of water to the tank, seal it up, turn on the air pump, if the pump shuts off check the pressure at the air nipple with a tire gauge. 20+ lbs pressure your good on the tank/pump. If the pump won't shut off within 10ish minits start looking for system leaks of either air or water. If no leaks and good pressure in the tank and the pump continues to run, probably the high pressure limit cut off switch on the air pump is bad. If no leaks and very little pressure in the tank the pump is bad, or possibly the air line between pump & tank is kinked or pluged.. Cold hard fact is the air press system was less than ideal when new. I only kept the one in my Catalina 'cause the rig is 100% original. Otherwise it would be a common on demand water pump, far less hassle.