recurring loss of tire pressure

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my left front tire has been losing pressure in it for a couple months now. it's gotten to the point where i have to put air in it almost every other day and it's getting old. it never gets completely flat and i make it a point to find an air pump when i notice it(i've stopped paying attention to the 'check tire' light as it's almost constantly on). any tire experts know what the problem could be? is it the tire, valve, seal? it never seems to get below 12-15psi, but i know it's not safe to drive with it like that. i've been filling it to the recommended pressure on the door.

the tread is still pretty decent on it and i'm afraid that if i go to a tire place they'll tell me automatically i need new tires(i work in sales too).