Vent normally will hold a slight vacuum. Why? It is a "jiggle" type valve, kind of like on a pressure cooker. Vibration while driving and the normal build up of pressure as the oil heats up will force air out the vent. Stop moving and the valve will seal off the opening to keep stuff like water and mud out. Normally OK, but if you are stopped in deep water a decent vacuum can build up and possibly suck water in past seals if any are weak:

http://www.4crawler.com/4x4/CheapTricks/WaterProofing.shtml#Breathers

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