Need help with P0442

A year ago I got the dreaded dash service light and P0442 scanner code. I replaced the gas cap with a new OE Toyota one. The problem went away for a couple of months, then returned with a vengeance. I took it to a local shop who got nowhere with it... now I can reset it all I want and the light and code come back right away.
Today I did a DIY smoke test, admittedly not knowing what I'm doing. Here is what I did:
I pulled the hose going from the purge valve to the air filter box off of the purge valve, and introduced smoke through that port on the valve. Smoke started coming out of the filler neck area, and on closer inspection it was from two small vent holes under the gas cap, visible from underneath the truck.
I'm not sure what this means. Can someone smarter than me tell me if this was a useful test, and what it might mean? Also, more info about the correct solenoid positions for doing a smoke test would be greatly appreciated.