Last can of spray I carried for 2 years (a xmas present from my wife). Decided to test it out and it was empty. I often think how silly I would have looked Holding a out as a bera charged waiting to get mauled with my arm out. Test it every year they don't last forever.
I don't bother now, I've had hundreds of bear encounters. Common sense and being alert has helped. I like the idea of making noise as I walk. MInd you, there was a salmon stream a trib of the Skeena where I was subject to charges so close I could smell the big buggers breath. If I hit that stream again it will be with a 12 gage shotgun and spare shorts.
Must tell you about a trip I was was on to a lake in the interior of BC with my brother in law. We where driving out of a lake in the evening and almost hit two people writhing on the ground in pain. It seems they saw a bear and let loose wiht some spray. The wind wasn't in their favor and punched the whole lot into their faces.It took all of our water supply to douse there eyes with and some eye wash so they could stand. I tracked back from where they where to see where the bear tracks where. It was a good 75 yards. more that effective for spray.
I think your best bet is to buy spray from a reputable outdoor shop. They have good volume in the stuff and it won't already have a shelf life to contend with. Get a holster, it's useless in your back pack. Most bear encounters come with only seconds or no warning.