Holding Tank Filling Up

[Badsanta]

The unidentified hose is your problem. You have to follow that.

[Scrimshaw4]

It definetly sounds to me like it's blocked tank venting. The only source of water filling the tank during pump out would be the result of a vacuum drawing water thru either the head intake or an overboard discharge line. Close both of these thru hulls before pump out. If the head drains during the pumpout then blocked venting is the most likely culprit. This could happen anywhere in the line from the tank to the vent itself. I'd recommend disconnecting the vent line at the vent and then, using your dinghy air pump, see if you can blow air back thru the line to the tank with the pumpout fitting open and then thru the vent itself. The airflow should unrestricted.

[peghall]

It definetly sounds to me like it's blocked tank venting.

I'd bet real money on it.

The only source of water filling the tank during pump out would be the result of a vacuum drawing water thru either the head intake or an overboard discharge line.

That's just not possible, 'cuz there's no way for suction that empties the bowl to draw water out of the intake because the only way intake water can get into the bowl is through the holes in the channel in the rim. So the most the pumpout can do is empty the bowl...followed by a hissing sound in the bowl as the pumpout tries to pull air into the tank from any possible source...and a joker valve that's new enough to still close tightly might not even allow that.

So I'm betting that the pumpout is pulling a vacuum that will only allow a small amount to be pulled out. Or... if the discharge fitting is on the top of the tank with a pickup tube inside the tank, the tube--if metal--is full of holes...but I've run across a few plastic tubes that fell off the fitting.