On Tue, 04 Jul 2006 22:38:40 +0100, Richard wrote:

Situation: petrol tank for my bike is shaped like an inverted 'U'.
Very clever in that the fuel pump is in one side of the U and there is
no connection between the bottom of the two halves.

Simply link both sides with a U pipe. No valves or taps needed. Provided
the pipe is big enough, then flow through this link pipe will drain both
sides equally, leaving you with a trivially small undrainable volume
that's merely the contents of the pipe.

But motorbikes generally tap both sides anyway, and have a changeover
tap. This gives you a reserve available only by switching over which is
the volume of one isolated side. Cleverly this reserve is also
"automatic" and you'll still have a reserve next time, even if you
forget to switch it back when you fill up.