Flat tyres frustrate everyone and they always seem to occur at the worst possible time. Your first reaction might be to change the tyre and have it repaired, but that might not be necessary. Your tyre might just have a faulty valve stem. The valve stem provides the means to fill the tyre with air. If it fails, it can begin to leak, allowing the tyre to go flat. Checking and replacing a valve stem is a relatively easy process you can do at home, saving you time and money.
Jack up the wheel. Put on goggles to prevent possible injury. Fill the tyre with air again. Spray soapy water into the valve stem. The valve has a leak if you see soap bubbles forming at the end of the stem. Press the valve stem in to let all of the air out of the tyre.
Insert the forked removal tip of the valve core tool into the valve stem. Rotate the valve core tool anti-clockwise to remove the valve core from the valve stem.
Use the threaded tip of the valve core tool to clean the interior threads in the valve stem.
Insert a new valve core into the valve stem. Insert the removal tip of the valve core tool and screw the valve core into the stem. Fill the tyre with air.
Place a valve cover on the valve stem. Lower the tyre and remove the jack.