More info.

I checked the hoses. They all seem to be OK. Even the one from the canister to the purge control valve. It's a bit dry but doesn't have any leaks.

I checked the voltage to the valve when the ignition is on. Shows 12V solid. I checked the valve itself with an external power supply. Works fine. Resistence on the valve was between 30-90ohm. As specified.

I checked the power to the purge flow sensor. 12V solid. I also checked the 5v current from the PCM. It's fine.
The sensor itself looked to be quite new so I checked the papers I got from the previous owner and found out that it was replaced 6 months ago. I tried to blow some air into it while measuring the resistence. It didn't change from 40ohm. I think this is the sorce of the proble. I will try to replace it tomorrow but I'm having trouble finding the right part. I have the part number off the one that is in the truck but I can't match it to anything on Motorcraft's website.

I'll let you know how it went tomorrow.