Freshwater Tank won't drain

Took our used WC out for the maiden voyage and things went pretty well. I did have trouble draining the freshwater tank when leaving. I turned the petcock and only got a dribble. Hmmm. Figured there might be a problem with the vent, so i checked it for mud daubbers or obstructions (none) and routed the vent tube up (done). Still not draining. Hmmm. I don't know why, but I turned on the water pump and lo and behold, as long as the pump was running the tank would drain.

In checking the lines I removed the tee that comes from the tank, with one side of the tee going to the drain and the other to the water pump. With the tee removed I could see a hard pipe inside the tank that was closed off. My guess is that this is some sort of block to prevent city water from going into the tank. Or, it may be a safety feature for the pump to keep it from draining too low and losing it's prime. I'm not really sure what it is and if it is even working correctly.

Has anybody out there experienced a similar problem and encountered the tank-pipe I mentioned? By the way, the water pump is running but no longer pulling water. Could use a little (maybe a lot!) of help.

The valve to prevent the city water from filling your fresh water tank is in the pump. If you have the drain for the fresh water tank open and turn the pump on, you will just suck air. The pump will prime itself with the drain closed and a faucet open. I don't know what this pipe inside the tank is that you are seeing. You didn't mention if your pump was drawing water before you tried to drain the tank. If it was working correctly, your blockage must be between the tee and your drain petcock.

The pump had been working and stopped unexpectedly. I checked for any blockage between the tee and petcock and it is clear. Will try later to open a faucet and see if the pump will prime. Thanks for the reply.

My valve is a back flow preventer valve hooked to the city water inlet.