Jabsco heads always worked fine for us. Every 5 to 7 years you just toss them, and install a brand new one which works fine for the next 5 to 7 years for less than the cost of a set of o-rings for many other marine
heads. There's nothing wrong with "throw-away items", if you are prepared to throw them away from time to time, and I'd sure rather throw away something like that than rummage around in its disgusting innards trying to rebuild
We've had great luck with them -- flush a little olive oil
down them from time to time and they just keep on flushin'.