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
