I think I posted this incorrectly before. The gang at the Catalina 30 blog directed me to this site and specifically to the Head Mistress. I have had an '89 Catalina 30 for 2 years. Last year all was well, except the waste vent clogged, but that was easy to fix. This year started out well, but now the head is doing weird things. Up to now, the bowl has always been dry, even when heeling. Earlier this summer I arrived to find the bowl was full and overflowing on a very hot day. Since my boat partner had a party the weekend before, I assumed tank just needed to be pumped out. Did that, plus the nasty cleanup, and it worked for about a month, then bowl backed up to half full (think this might be where sea level is). Pumped out the tank again and within a week, water is back in the bowl again halfway up. When I pump the toilet down now, I can see the bowl filling back up - takes about 2 minutes. The literature left behind from PO says my head is a Par, but that might not be current. The base of the head is at or slightly above the level of the top of the holding tank. I have no anti-siphon loops on the system. The y valve is in the correct position and all the thru-hull valves (raw water intake, macerator waste out, and direct waste out) are shut. We don't use the bay water in the toilet - just squirt from the sink. Is there an easy way to trouble shoot the location of the problem? Can that circular plate in the top of the tank, where the vent hose attaches be removed so I can see into the tank? (this of course would be after pumping down the tank again!) Any help would be greatly appreciated. Thanks, Irene