You might not need the whole stem assembly. Inside the supply line there is a little rubber washer on top of a spring. I took out the old spring and rubber, took them to the hardware store, found a match, and reinstalled after a bit of cleaning inside the valve with a wire brush and toothpick. Worked like a charm for about three and a half bucks.
My recollection is there is also a bit of plastic between the spring and rubber, so watch for that too. Getting them in the right positioning was a bit of a trick, but a blunt pencil helped stack everything back in place. I've done this twice in the last three years, once for each valve. Pop the old stem back in and Bob's ur uncle....
Cheers,
Max
