GS's never asks for RES?

[Kerry]

No, not at all.  This all happens on the fuel tank side of the (frame-mounted) petcock.  The carbs don't have any clue about which fuel tank line the fuel is coming from.  As long as the petcock keeps sending fuel, the carbs are happy.

The only difference between the ON and RES lines is the level of the "drain" opening inside the fuel tank.  The ON and RES positions on the petcock are both vacuum-actuated, so there's not even a difference there.

PRI is the only really "different" position because it's gravity-fed.  I have never had this confirmed, but for my experiences to make sense the PRI position must pull fuel from the RES fuel line.  But again, as long as the float bowls are full, the carbs couldn't care less about what happens upstream to keep them that way....