I installed a Lavac Popular last year, and put an anti-siphon valve in the fresh water loop, instead of the little Lavac valve, taht they send with the head.
However, I don't get any new water in the
toilet at all. It will pump out what is in the bowl, but doesn't bring in new water, with the lid down. There is some suction, as the lids sticks down, but not for long - maybe 5-10 seconds. I assume that your lid is vacuum sealed for longer than that???
I think I may have a small airleak, where the fresh water inlet fitting scews into the back of the bowl, as it feels a bit loose. I will have to take the hose off though, in order to tighten it!
Got through last year by just bringing in buckets of water from the
cockpit, to fill the bowl - had bigger problems with the
boat, so never got around to fixing the airleak!
One complaint about the Popular model - the plastic clamps that hold the seat, and lid onto the bowl are
cheap - and loosen easily / don't keep the seat / lid on solidly!