If you think you have a leak in a tire, check out the valve stem on the tire before you take it to a mechanic.
To determine whether the core of the valve stem is leaking, remove the cap and use your finger to apply a little soapy water around the valve. If bubbles form, the core is leaking and needs to be replaced. You can get a new core and a valve-repair tool at any auto-parts store.
Here's how to replace the valve: 1) Carefully jack up the car so you can let the air out of the tire. 2) Unscrew the valve using the repair tool. 3) Safety tip: Be careful! The valve can fly out just from the pressure in the tire. 4) Screw the new core in place and refill the tire with the proper amount of air. Then retest to make sure the new valve core is functioning properly. If it is, put the valve cap back in place.