I have had the stem problem before. The metal part of the stem starts to corrode and releases from the rubber causing the whole thing to slip out. The best solution that I could find for it was GOOP. I took the stem out cleaned it up with some sand paper. Made sure that most of the powdery stuff in the rubber part of the stem was scraped out. Glued the metal stem in careful not to get any glue in the inflation valve. It has worked for over a year. I did contact the lawnmower shop to see what they can do for me. Maybe the next time I lose a stem I will put an innertube in.

Later,
Dan