I'll bet your canister is fuel logged and you likely have pallets in the system which will need to be blown out with air.

 

I am not sure if I understand this.

However let me add that several years ago and I believe it was prior to having these codes pop up almost on an annual basis, I did have a fuel tank overfill. For whatever reason, the gas station pump did not click off until gas shot out and all over my leg. At that point something like 16+ gallons had been pumped in when I probably only needed about 14 to fill up.
Is this related to any of these problems?

 

See if your canister is full of fuel.

 

If you can pressurize the system a little and spray things with Simple Green you'll find it.

 

Hey, good idea! Had a shop smoke test it, but as I feared, they found no leak. I only learned later that at least some of the smoke is fluorescent, so I didn’t ask if they used a black light. (Even if you can’t see the smoke, it might leave visible smudged at a conveniently located leak.)

I’m making an appointment today with a different shop, ostensibly to just replace the rusty evap line, but hope I can motivate them to track this down if that’s not it.

 

and how would I go about finding out if the canister is full of fuel? My accidental fuel overfilling was years ago. Wouldn't it have dried out by now or would the overfilling have permanently damaged the canister?

I'm definitely a shade tree mechanic here and layman's terms would be greatly appreciated. Autozone has the vapor canister for $190 and the Vent Valve solenoid for $42. If replaced, I imagine they would be done in pairs since they are attached to each other?

Rich

 

Test the vent valve solenoid, it should be closed and you shouldn’t be able to blow air through it.
the canister should be open

 

You have a hard time starting after fueling correct? Purge valve is bad. Canister may never 'dry' out if the system is not venting or purging correctly.

 

Would a bad Vapor Canister cause a Purge Valve to keep failing? I have replaced the purge valve 3 times in the last 4 years.

 

Yes. The system will have pellets in it and they will keep the purge valve from closing. If you replace the valve without blowing out the lines your new valve will fail in short order.
You likely need the canister and purge valve replaced and blow out the lines.