I never knew that about the venting either. Getting the thing off the nipple was always a battle until I discovered these things. The are superb and and making greasing so much easier

http://www.alemlube.com.au/product/quick-release-grease-gun-coupler/

Cheers John
2011 4/4 - "Elrond"
2007 Elfin C3 Clubman - “Gimli”