02 Silverado - SES - P0440 Code - EVAP System Problem

Hi Guys, I have an 02 Silverado Z71 with 48K mi. on a 5.3L engine. I recently started getting a SES light which decoded to code P0440 (Evaporative Emission Control System Problem). I tried clearing the code, but it would consistently return a few days later. I understand that the most common cause of this code is a fuel cap that fails to hold pressure. So, in an effort to fix it fast and cheap, I replaced the fuel cap. Now the code returns less often, maybe every week or two instead of every day or two, but it still eventually returns after clearing it. I understand that there are potential issues with the EVAP vent solenoid, and that this problem may be covered under federal emissions warranty requirements. Can anyone advise me further on the nature of this code / problem and how to best get it fixed? The truck seems to run fine otherwise, just getting sick of seeing the SES light. Please post replies to the group. Any information is appreciated. Thanks...