This is rather belated, Marc - if you ever get to read this - but frankly, I still find it hard to believe that pumping out higher up was your answer really. The small amount of back-pressure from the few inches under water
the outlet was is just so small any pump should have overcome it. Your system sounds in principle very much like mine, after I was forced by regulations
here is Queensland
, to install a holding tank
in my Clipper 34. I use a TMC macerator pump for thru-hull pump-out from a bladder holding tank
in my for'd bilge
space, which was supposed to self-prime to up to one metre, but never has. So if I go any length of time between pump-outs, I have to prime the macerator pump by reversing flow for a few minutes by pumping water
from the toilet back down to it with some juggling of taps and valves - it then works well. I suspect your pump was suffering the same problem, and somehow when you changed your outlet, the setup holds fluid in the line better, keeping the pump primed - or the pump has ended up lower than it was before, or both, or you had a kink somewhere in the original outlet? Am I right? As you found, the anti-siphon valve is only relevant to stopping back flow in, and easily stopped by a tap in the line and/or turning off the seacock. I use both, as that tap is also what allows my priming diversion.