CEL code P0440

[Clark73]

1996 Rodeo 3.2L 4x4

I'm having trouble finding where this problem is coming from. I replaced the gas cap but the code just came back a couple weeks later. I followed the evap hose from the gas fill tube to the canister and couldn't find any problems. Then I followed the second hose to the engine and still didn't find any problems I could recognize. I really don't know what to look for or where the problems usually occur. Along the way down the evap hoses though I found some parts that I couldn't identify. Next to the canister there was a little black box that looked to have electrical parts. Then when I got to the engine there was another little black box shaped thing with the hose going in and out of it. Also about 10 inches before that there was a little valve like the valves on the tires. Could these be where the prob is coming from?