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I have never really considered the need to relieve positive pressure from a hot tank.
I guess there could be some pressure increase on a very hot day, but this would be relieved as soon as you rode off and burnt a little fuel.
I've never owned an injection bike but I guess these would need a vent to relieve pressure from the pumped fuel return.
No such issues on a carby, where the main issue is relieving negative tank pressure as the fuel tries to flow out.
My own interest in the valve relates to the XT500 tank that I've fitted to my Dommie.
Here the filler cap is vented (breathes) via a spigot and rubber pipe which tucks down a hole in the steering stem nut.
My concern is to prevent fuel escaping out of this breather pipe if the bike falls over.
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