There was a rash of tire valve stem failures traced to a Chinese supplier several years ago. I got stung by this and I got really mad at having to get the leaky stems changed at a shop at $$ per tire. I decided to just buy a box of stems and replace them myself but how to get at the inside of the tire? Easy. I placed the original equipment bumper jack on the tire right at the stem and jacked the bumper up, thus pressing the tire bead off the rim just enough to swap out the stem. An inexpensive stem tool pulls the stem into the hole until it seats. Then I bounce the tire around until the bead re-seats while pumping it up with compressed air. A difficult tire that won't re-seat can be persuaded to by wrapping a stout rope or chain around the tire and twisting it tight with a tire iron while pumping it up. Best part is- you don't disturb the balance.