Sorry for waking up this topic but I've ran on this issue.
It seems the vent valve accumulates chalky deposits which avoid it to
seals properly when closed. I've noticed only when a lot of water was on the
bilge already. Hopefully It was on the marina and I was using sweet water (this flush can use salty water as well but I only switch that circuit on when at anchor).
It happened the same with the solenoid valve itself 1 year ago or so. This one almost caused a flood to other rooms of the
boat as when I've noticed the head bowl was full and the
shower area was already getting full as well).
After a good clean, the solenoid valve is working since then without any issues.
The vent valve it's harder to clean as it's a sealed piece, anyway I've blown strong through the vent (which made some grains get immediately blown out) and let it stay for a few minutes inside a cup with some anti-calcification products. Seems to
work now but don't trust it that much as I can't inspect the rubber (and spring?) inside the sealled compartment.
I found it to looks a lot like a gas tank vent check valve for some jet skis with a external rubber seal. I've ordered one and I'll post a feed back as soon as I get it.
My vent valve:
The one I've ordered:
It seems I made a pretty good saving with this 2 services to the Jabsco solenoid valve. A OEM replace is ridiculously expensive in
Europe: