The vent is usually part of the bolt that holds the brake line tee fitting in place. Follow the brake flex line from the frame to the rear end, and you'll either find a loose-fitting metal cap on top of the bolt that holds the brake line tee on, or a rubber hose attached to the top of the bolt that ends up somewhere inside the frame.This allows for air expansion inside the rear end housing. (Lets water in, too, if you submerge the rear end!)
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!