how to trouble shoot exact problem.

What is your question?

Truck would not take gas. My nephew mechanic told me to pull the vapor cannister line from the tank to can off from can. I did. Truck takes gas now but slowly. Neph says i need new cannister. I start finding out there are purge valves and silinoids that would do this same thing. does pulling line from can mean it is the can ? How do i find the exact problem, or have i already, before i go buying 100 dollar cannister ?