I have a parts car that runs good. Would you consider a used one or go completely new? And why run good on the bottom 1/2 vs. the top half.
I would definitely try the used one first. I think the canister is mounted inside the quarter panel near the filler neck. So swapping it out should require jacking the car, removing the wheel, partially removing the fender liner, then removing the canister. For a 150 bucks, I'll burn an hour or two doing that.
As for why it works half full but not all the way, my half baked theory is that the vapor recovery system tries to move some vapor thru the system at engine crank, and, if the tank is full and the canister clogged, the system chokes on it thereby somehow preventing the engine from firing. Whereas, with the tank half full, there's enough elasticity (ie, compressible air volume) in the system to squeeze the "burp" of vapor into the tank without choking things on the other end.
As I understand it, using the solenoid actuated valves at each end of the car, the system attempts to recycle vapors from the engine end of the fuel system back to the tank rather than releasing them into the atmosphere, like we did in the old days
Beyond that, I don't know much about vapor recovery other than to not top off the tank after the pump stops like Dad did
(And like I did for the first 30 years of driving
[Edit]Thinking about this some more, there's a small hose that connects to the filler neck. I think, when the tank is topped off, that hose gets gasoline in it, which does indeed choke the system when it attempts to burp itself. After that hose clears itself (however that works) the hard-start problem goes away.