I had a flat tire , aired it up took to shop they fixed it said had a small nail. three days later tried to air it up to drive bake to shop. had a red rubber ring inside of stem. what is that for?
It's an O-ring to seal the cap to the valve stem. It belongs inside the cap. If you crank the cap on too tight, you force it into the valve core area. Just remove it and shove it back into the cap