When I bought my 2000 Carver 406, one of the first things I did was have the holding tanks pumped out. I noted that the forward Tankwatch III was not giving and indications while the aft tank showed empty when pumped out. I decided to used the forward head/tank only with RV Digest-It in an attempt to clean up the switches on the Tankwatch. After a short while, I got a mid-level light on the aft holding tank. The aft head had not been used. After watching this through several pumpouts and opening the forward holding tank, I've determined that my forward tank is likely draining to the aft holding tank by gravity. Having chased down the sanitary hoses, the only way I see for this to be possible is if the y-valve by the overboard discharge pump has failed and is allowing forward to aft flow. Has anyone ever heard of this? Should I just replace the y-valve? It doesn't turn anyway, but I never pump overboard, so I wouldn't need to replace it for that reason. Cost is not an issue - just hoping to avoid the dreaded onboard sewage spill.