I have had mine for years and the button kept falling off in the middle of bottling. The button ended up in the bottle and the beer would keep coming out, making a big mess and wasting precious beer. I was bummed because when the gun was working properly I loved it, when it failed, I was in a panic to get the beer to stop flowing.
So to solve this I went to the internet and found almost nothing about this. I tried replacing the buttons and it helped for a while, but before long it fell of again. Finally I found a short blog by a guy (that I can't find again) that came up with a simply and brilliant solution. Take the tab that holds the button and bend it up slightly past 90 degrees. This will cause the button to be pushed back into place each time the "fill valve" assembly is operated. Since I did this a year ago, I haven't had a button come off. I have attached a photo to show what I did.