Occasionally when fixing tires I find that the valve stem core is bad or loose. It's easy to diagnose and is a simple fix. An inexpensive tool and parts will make this a repair anyone can do.
Tools you'll need:
First put your safety glasses on. To check if the valve core is leaking make sure there is some air pressure in the tire, if not add some with a compressor or bicycle hand pump (you don't have to air it all the way up at this stage but you do need some air pressure in the tire). After you have some air pressure in the tire squirt a little soapy water on the end of the valve stem, if I lack soapy water I just spit on it. If it's leaking you'll soon see bubbles. You might want to squirt some soapy water around the base of the valve stem as well I've found leaks there as well but I'll cover that repair in another article.
You see bubbles, now what?
You'll need safety glasses, put these on they don't do you any good in your pocket, a valve core removal tool and possibly a new valve core. You can find all of these at your local auto parts store.
A clearer picture of the same, I don't recommend the plastic cap tool it's just there to show options you might have in a pinch there might be one on a tire.
With your safety glasses on first check for dirt or debris this can hold the valve slightly open. If you see dirt you can use the back side of a tire pressure gauge to bleed some air out of the tire, this will usually blow the dirt out some times fixing the issue. Check with soapy water.
If it is still leaking use your valve core removal tool to try and tighten the valve core, put the end of the tool inside the end of the valve stem and when you feel the tool seat on the valve core turn it clockwise. You don't have to turn it hard or force it turn it till it stops, I once had one actually start leaking worse when I tightened the valve core so I backed it out till it was barely leaking. Check with soapy water. If it's still leaking you'll need to change the valve core.
To change the valve core first drain the air out of the tire, if you leave the tire aired up the air pressure can cause the valve core to shoot out when you unscrew it this can be dangerous you don't want that to hit you in the eye. With the air pressure drained insert your valve core removal tool and turn the valve core counter clockwise, unscrew the valve core all the way and pull out. Place a new valve core into the tool and place the new valve core into the valve stem and turn clockwise, being careful not to cross thread it, until the valve core seats in the bottom and stops turning. Put some air in the tire and check the valve stem with soapy water. If there are no bubbles you have successfully replaced the valve core, finishing airing up the tire to the proper air pressure.
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