Transmission Leak

[VeeTwin60]

Whenever I let my truck sit for a couple of days to ride the Harley and save some gas, I come back to a puddle of trans fluid under the truck.  Now, I know that I need to replace the O-Ring where the dipstick goes into the case, and that is probably where it is coming from (or am I off-base in that assumption?).  But my main question is why does it not leak when I drive everyday (not a drop), but after it sits for 24+hours, it pours out?

[choptop]

I have the same problem with 2 of my trucks. If driven every day, no leaks, let is sit, and it leaks. Was told something about an oring in the tailshaft area, but I am not sure. Hopefully someone will shed some light on this.

[VileZambonie]

Gravity and viscosity. when the oil is in motion it actually seals by bonding together. At rest gravity wins