It is scary that it went flat without inflation of the vest. I think you have some bigger issues or need a new rebuild kit or change the entire vest out. I am redoing all 4 of my vest. I have one vest thats very old but I think it will be fine with a new inflator rebuild kit. I suggest you get very familiar with how these things operate. Take them apart and rebuild it. The scarry thing with one of these you do not want to find out it doesnt work when you need it. Here is a good source of information.
http://www.inflatablepfd.com/
http://www.inflatablepfd.com/