2000 Cherokee Sport EVAP leak?

My engine light came on yesterday, autozone computer says it's a large leak in the EVAP system. I had this same problem about a month ago, but the light went out when I replaced the leak detection pump. My mechanic pressurized the entire system, there are no leaks. I just bought a new gas cap, I have driven about 40 miles but haven't manually reset the computer (pulled a battery terminal off). If this doesn't work, does anyone have any suggestions?