I assume that if you have two tanks you have a valve to turn each one on and off so that when you run out of water in one tank, you turn that off and turn on the other full tank. If your pump keeps running it simply does so because you have a leak where the pressure will not completely build up. It is a demand pump. As soon as you open a faucet, the pressure decreases and the pump works. Turn the faucet off and the water flow stops immediately, but the pump will run until the pressure builds up to its max. If it continues to run, then something is keeping it from maxing out the pressure. This could be because your second empty tank has an open valve, or you have a leak somewhere in the system, which most likely is where there are fittings, or maybe even a burst hose. If you have a water heater, check it out to make sure it is bypassed as you say. I don't know why it would be. If the hoses are hooked up to it, then maybe that tank is empty or the valve is open on the hot water tank. You have to close that pressure valve then turn on your hot water faucet and fill up the hot water tank so that it eventually comes out of your sink faucet. Remember to also never turn on the switch for the hot water heater unless you know it is full of water. Otherwise you will burn out your heating element and that will have to be replaced. Also, if you have a hot water heater that is hooked up to a heat exchanger attached to your inboard, you must make sure your coolant water is full or you can over heat your engine. But, the engine side is antifreeze coolant and the fresh water pressure pump is your fresh water (two different systems).
