I've got a 1990 Sundancer 350 with...

[jimboc]

I've got a 1990 Sundancer 350 with a vacuum flush headsystem and  Sometimes after flushing  And bringing in some fresh water to refill the bowl and then putting the foot actuator back in the neutral position so it is no longer bringing in water, sometimes the vacuum will suck the water out of the bowl until then you hear a hissing sound.  Sometimes it occurs after someone flushes toilet paper which gets caught between the rubber seal and the curved sealing valve,  So I will take my finger and run it along the entire edge of that seal to clearany debris and it will seal but not every time. sometimes depends on the position it will continue to leak water and then hiss air any suggestions

[Rick_CJ]

We also have the vacuflush system. It’s great when it works. We don’t put paper down so we don’t have that issue but what I have found is when you flush, don’t ease the pedal back up. Take your foot off at the bottom so you get a slam shut on the rubber seal. Also as you mentioned, you need to keep the toilet and gasket clean.

[No_thank_you]

With it open wipe the rubber seal with a vegetable oil.

[Airwaves]

I replaced the rubber seals in the bowl when I bought my boat 3 years ago, as they were quite inflexible and would not seal. Not hard to do.