On May 19, 5:27 pm, quietguy wrote:
There were no cracks or other obvious
flaws but both gaskets were equally bad at sealing; that's why a
simple cap-swap didn't change anything. It seems that the left tank
just isn't as sensitive to venting/sealing problems as the right. To
seal well, even with good gaskets, the caps have to be difficult to
screw down tight.

The LH tank vents directly outside, while the RH tank vent into
the left, so a leaking RH cap would hold back the flow in flight
somewhat due to the small resistance of airflow through the vent
crossover. But leaking caps still don't explain the lack of crossflow
overnight; only a totally plugged vent system or plugged fuel lines
would do that.
Those cap gaskets compress with age and get hard, and won't seal
unless they're really clamped down tight. New gaskets fix that, but
they can be so tight that the cap doesn't get screwed on all the way
and can come off in flight. I painted red stripes on our caps so that
the stripes are lined up fore-and-aft when the caps are all the way
on. And I use a little tiny bit of Dow DC-4 silicone grease between
the cap and gasket so the cap doesn't have to grind the gasket against
the filler neck; the cap goes on really nice after that.

Dan