Cleaning the Evap vent valve and have question

[bradjh]

took vent valve off and cleaned it good, but i have a question what is that black plastic box that is about 7" by 9" and directly over rear axle on driver side behind gas tank? all this time I thought that was vent valve and could never get it off>> but found vent valve up further..

[HenryJ]

That is the evaporative emissions system charcoal canister.

[bradjh]

Is that something that can be or needs to be cleaned?

[HenryJ]

No. It is a sealed unit filled with charcoal. It filters / absorbs the fuel fumes and prevents them from entering the atmosphere. As long as the vent valve filter keeps the dirt from filling it, you should be fine. In a severe overfill situation they can become flooded and require replacement.
I have replaced a few on the Ford F150 trucks. Those use no filter and fill with dust/dirt. I have not seen the same problem in the Chevrolets.