Sometimes a valve stem can start leaking because it gets weather cracked or bent until broken. Test this by inflating the tire and wiggling the stem while spraying soapy water on it, if it leaks from anywhere but the fill hole it is bad. If only the fill hole leaks then this Instructable is not for you because you probably just need to change out the valve inside the stem.

Step 1: Things to Get:

1) New valve stems (available at Walmart or any automotive parts store for a couple dollars).
2) Valve stem install tool (also available at Walmart or auto parts store for a buck or two); does two things:
a) valve remover.
b) valve stem puller.
3) Long handle pliars (channel locks work great).
4) soapy water in a spray bottle. 409 or dish soap + water, or etc... just needs to be very soapy/slippery.
5) A car jack.
6) A few blocks of wood like 4x4s. about 2 ft long.
7) An air compressor.
8) A tire pressure gage.

Step 2: Prepare the Tire.

1) Remove the tire from the vehicle.
2) Deflate the tire and remove the valve from the valve stem using the valve stem tool.

Step 3: Push the Rubber Tire Down to Expose the Valve Inside the Rim.

CAUTION: wear safety glasses and a long reach rod to control the jack. The forces applied can become dangerous if the jack slips or if the tire slides.

3) Set up the tire as shown under the bumper of a heavy vehicle.

A) Use blocks opposite the jack to keep the tire from flipping up while using the jack to push down.

B) Use a jack to force the rubber tire down off the rim near the valve.

TIP: You may need to do this to the left and right of the stem first to loosen up the bead lock before it will break loose.

Step 4: Remove the Bad Valve Stem.

4) Use pliars to reach in and grasp the back of the valve stem.

5) Cut the front of the valve stem off if needed to get the back to come loose.

CAUTION: Never place any part of your body in between the rubber and rim. amputation could occur if the jack popped loose. Use pliars not hands!!

Step 5: Install the New Valve Stem.

6) Open the new valve stem (available at Walmart or any auto shop).
7) Remove the valve stem cap.
8) Remove the valve.
9) Spray the entire stem with soapy water to make it slippery for easy install.
10) Grasp the valve stem base with long pliars.
11) Hold the valve stem pull tool in your other hand.
12) Use the pliars to reach the valve stem into the rim valve hole.
13) Screw the valve stem pull tool onto the stem from the outside.
14) Gently pull the valve stem tool with increasing force until the stem pops into place.

Step 6: Re-install the Tire on the Rim.

15) Remove the jack and wood blocks to get the tire out. The rubber should naturally push up toward the rim, but will not go all of the way back.
16) Replace the valve into the valve stem.
17) Spray soap all around the tire again to make it slippery around the rim.
18) Start inflating the tire. Keep hands back as much as possible because the tire will pop into place on the rim with ALOT of force.

Step 7: Re-inflate the Tire to Recomended Pressure.

19) Once the rubber is back where it belongs on the rim, use a tire pressure gage to adjust the tire pressure to manufacturer recomendation. 50 psi max in this example. (I chose to stop at 45psi.)