My guess...
Your holding tank vent is partially blocked, causing the tank to pressurize and create backpressure when you pump the head. Just enough air is passing through it to relieve the pressure between each use of the toilet, and to let the waste move a little further each time you pump. If I'm right, you don't feel any resistance the first time you pump the head, but you do when you pump it again after part of the contents back up. So the first thing to check is your holding tank vent thru-hull...that's where 99% of vent blockages occur. However, if you've oveflowed the tank out the vent--either due to negligence or heeling, waste may have collected in the hose--in which case, just replacing the vent hose may be easier than trying to clear it. Just don't try to pump out the tank until you're positive the vent is completely open. If that's not it, you have a partial blockage in the discharge line...if there's a vented loop in it, the best bet is between the toilet and the top of the loop. If it's waste, you may be able to dissolve it by pouring a healthy dose of Raritan K.O. into the head and flushing it just once...then let it stand at least overnight. In salt water, sea water calcium carbonate build up in the head discharge hose is very common...I've seen it reduce a 1.5" ID hose to less than half an inch. It happens a lot faster in warm water than in cold, but that could be your problem on an 11 year old boat if the hoses haven't been changed in more than 2-3 years. It can be prevented by putting a cupful of undiluted white vinegar (use the dry setting) down the toilet once a week when it can stand for at least 8 hours. The cure is a bit more involved. White vinegar will dissolve the buildup, but it takes repeated applications--several days--to completely clean out a hose. Muriatic acid will do it in one or two applications but you need to be very careful and follow directions when handling it. So the first thing to check is your holding tank vent thru-hull...that's where 99% of vent blockages occur. And don't try to pump out the tank until you're positive the vent is completely open. Next, pull the hose off the toilet discharge fitting and see what the inside of it looks like (put a pan under to catch any spill)...whether it's open or has build-up in it. If it doesn't, pour about a cupful of K.O. down the toilet and wait 24 hours. And if it's none of the above, get back to me. Btw...If more than just liquid is backing up into the bowl, your joker valve is LONG overdue for replacement, too. In fact, when--if ever--was the last time your head was rebuilt or replaced?
