Thanks everyone for your replys, my wife and I are new to the cruising style boats. Pascal I thin you hit the nail on the head with the filter, I looked inside the thru hull and can see a black something that I presume is the filter you speak of. I will be checking into it more this weekend. I now know it isn't supposed to have a vacuum when empting the tank. I gues that filter has been plugged for a while, we have only been the boat owners for a month.