I'm having an issue with my foward sump tank backing up in one of my VIP staterooms and flooding the head, Also it happens when my house air goes below 50psi
most sumps have a screen at the inlet to catch stuff coming in from teh showers... they need to be cleaned once in a while. or the pump/float switch may have a problem but usually it woudl cause the sump to overflow in the bilge rather than back up, unless the sump is sealed. obviously, i'm assuming you're talking about a grey water sump, not a black water tank.. and head refers to the shower drain or a drain in the floor of the head (as in bathroom, not as in "toilet")
It is probably one of the duck bill valves. The sump is a bit unusual on the Lazzaras, it uses compressed air to force the water out of the sump tank. It is a Lazzara design. Any time you loose air pressure, your sump will not work. On the 106 I used to run, we replaced it with a more typical sump system. Wait until someone puts bubble bath in the tub, that will be a whole different issue for you.
Ah yes, the air operated sumps on a Lazzara. Something you guys don't have to worry about anymore. Hope you two and the boat are doing well.