It sounds like the vacuum switch on the side of your key switch is bad. It is only 2 screws, but a pain to get at them. Last time I removed it, I ended up pulling the switch shaft off the column to get at them, and turned out my switch was fine and I had the 2 vacuum tubes reversed. I suggest that you test your shut-off valve first using a hand pump. The vacuum switch is fairly cheap (~$25). As far as how the engine shut-off works, there are maybe a gazillion posts, so search and read, nobody is going to spoon feed it to you.