Peggy, I took your advice, and instead of overhauling my 12 year old Jabsco 29090-2000 Manual Head, I simply picked up a new one on sale at a popular national marine store that shall remain nameless. My old installation had no vented loop anywhere on the intake hose (what was Hunter thinking, there are no vented loops on the entire boat?), and the toilet is installed just below the water line. The Jabsco instructions say "if the toilet is...below the water line...a 3/4" vented loop must be installed in a length of hose connecting the flushing pump to the rear of the bowl...positioned eight inches above the waterline at all angles of heel..." A short, direct line comes already installed on the head between the pump and the bowl. It makes more sense to me to locate the vented loop between the seacock and flushing pump instead of between the pump and bowl for two reasons: 1) a loop between the pump and bowl offers no protection if the hose fails between the seacock and the head, and 2) if I replace the supplied line with a vented loop, the only place to put it will be within the head compartment, on the bulkhead wall behind the toilet. But if I install it between the seacock and the pump, I can hide it out of sight under the sink cabinet on the inside of hull. Anything wrong with doing this? By the way, I've been using K.O. and C.P. since last summer, and when I disconnected the hoses from the head and the holding tank, there was absolutely no odor at all! Amazing! Thanks, "Headless (temporarily) in Seattle"