I'd guess your head is clogged--
...And with either something other than toilet paper (perhaps a tampon?) or a LOT of it.. The head is pushing water out against the clog, but can't get past it, and and because the seacock is closed, residual sewage left in the discharge hose is being pulled back into the bowl...which also means your joker valve needs replacing--AND--you and/or your guests haven't been flushing the toilet completely. (Read the article in this forum "Flush with Success") Toilet paper--unless it's the wrong kind or a huge wad of it won't clog a head.--at least not one that doesn't need rebuilding so badly it can barely pump water and solids, much less anything else. You can't flush "premium" brands...they're impregnated with extra fibers for strength and creams for softness that keep 'em intact for a week...but if you're using "marine/rv" toilet paper, or the cheapest "no name" institutional grade from the supermarket (which is the same thing for half the price), and using it sparingly, there's no reason not to flush it. If you're in any doubt as to whether a toilet paper is suitable for use aboard, tear off a sheet and put it in glass of water overnight. If it's still an intact--or mostly intact--sheet in the morning, it's not suitable...but if you stir the water and all that remains is a "snow storm," a marine head will handle it with no trouble. Just use it sparingly. Back to your problem...you have an unpleasant job ahead: taking the pump apart--and maybe the whole system downstream of the toilet--to find and clear the clog. And as long as you have it apart, rebuild it! Peggie
