My boat is a 1995 Carver 325 aft cabin. I bought it in October 2006. It is in salt water on the East Coast. We have been using the head fairly frequently over the last 6 months and last week I went over to the marina pump out station to have the holding tank pumped out. The operator could not pump it. He said that he could not draw a vacuum on it and either it is empty, or there is a valve in the wrong place. The head is still working fine. The set up is a Crown Head with electric flush and macerator. The pipe from the head goes to a Jabsco Par Y valve with is set to deliver the sewage into the holding tank. The valve handle has been removed. The other output from that Y valve goes to an overboard discharge. The holding tank is a 20 gallon tank, and the output from it goes to a second Jabsco Y valve which has an outlet for overboard discharge, and an outlet to the deck discharge fitting. The valve handle is set for the deck discharge. The holding tank is set way down in the hull below the generator and it is not easy to access or see how full it is. There is a vent pipe from the tank to the hull. Any ideas why the deck pump out is having this problem?