We recently needed to upend the tandem to get it through a gate and were horrified by the amount of rusty water that came out of the captains seat tube.  Investigations showed that the drainage hole in the bottom of the seat tube was blocked by grease so the tube was just filling up.  On top of that due to the type of eccentric fitted there's nowhere for the water to drain out anyway and also there is no drainage hole in the BB shell.

Worse however was the stoker seat tube. Here the water was trapped by the bottom bracket, no drainage hole again, so the shell and the seat tube were full of black oil like water. I'm amazed the bottom bracket still felt OK.  Due to the amount of water in the stokers seat tube corrosion was forming between the shim and the seat tube and I had to fashion a heath robinson seat post puller to get the seat post out of the frame. I will need to take the frame to a bike shop and get the seat tube reamed to clean it up.

We transport the bike upright on the roof of the car so it is likely that driving rain found its way in through the slit in the shim and the pinch slot in the frame.

Moral of the story, check for water ingress on a regular basis and if you can prevent the water getting and provide an exit path for it if it does.

Russell