Thanks Peggy. The discharge hose (toilet to holding tank) is the correct size (1 1/2). I inspected the hose set-up more closely. Fresh water comes in the lower 3/4 hose on the pump and the upper 3/4 hose runs out into a standing locker to a vented loop and returns to the head at the upper back of the toilet bowl, apparently feeding the bowl when the switch is in the wet position. Yesterday, I removed the vent hose that runs from the holding tank to a vent fitting in the transom and flushed it with fresh water from a dock water hose. Water flowed freely from the vent onto the transom. I then pumped-out the tank and when I was replenishing with KO and freshwater to start anew, this upper 3/4 hose again showed resistance when pumping the bowl dry. Everything flows freely when the pump is in the wet position, including emptying the bowl contents. I'll try your suggestion about replacing the dry/flush valve. Thanks again.