[Uhehner]

The vent is plugged or the charcoal filter if you have one. When you try to add waste to the tank with the vent plugged the pressure builds up in the tank and forces the waste back out the easiest path. Use a dental pick to remove the screens in the overboard vent. As a test, leave the waste pump-out cap loose when you flush and everything should work properly. With the vent plugged you cannot pump the tank empty either. You may even collapse the tank. Been there, done that. Capt. George