Water pump slows to a trickle

Question for you. When you winterize do you have a suction tube that you use to draw antifreeze straight out of the jug? If this is the case the valve you use to select where you draw fluid from may be in the wrong position allowing you to draw air in from one side and water from the tank. For some reason mine has a 3 position valve on it. One position draws from the tank, one draws from the suction tube, and the 3rd one draws from both. If in the 3rd position with all faucets closed it will eventually draw enough water to build pressure and shut off the pump. But once there is a demand for water the pump can not keep up due to the air being added to the equation.