Just one addition to William''s thorough reply: the intake line
''s antisiphon vent needs to be between the head (toilet) pump
and the bowl connection point. If you hook it up where you may think it goes - somewhere between the intake thru-hull and the head (toilet) pump
, it won''t work.
The Forespar a-s vents seem to last indefinitely and I like the fact you can ask them for a replacement duck-bill or two (the little one-way rubber valve that gives the fitting its anti-siphon capability) which they usually will provide you for free.