Loss of pressure in a sound tire can be from permeation through the rubber or from an old valve stem. If your tires are close to being replaced, consider nitrogen inflation. I got tires from Costco and they inflate them with nitrogen. It's inert and won't oxidize the tires as air will and nitrogen won't permeate the rubber as much as oxygen in air. I check my tires every 1000 miles or so and for the past 2 years that I've had the nitrogen inflated tires, have had to put in more nitrogen only when the pressure dropped a pound or so (twice, as I recall).