84 z28 305 quadrajet. Can someone tell me why the tvs that controls the charcoal canister has a bleed hole through the stem of it. From what I've read, most tvs can be tested by a hand vac to see if they hold a vacuum. I thought I had a stuck tvs so bought new one and same thing. Cut old one open and found the bleed hole through stem. This is the tvs that screws into the intake manifold and is opened when coolant reaches certain temp to keep fuel vapors from canister being dumped into cold engine. thanks.

 

You may never know EXACTLY why unless you talk with somebody who had a hand in designing the system (unlikely).

This is an educated guess ONLY: To let any residual vacuum between the switch and the canister purge valve bleed off to prevent holding it open when it shouldn't be. Perhaps they don't trust that the TVS seals off vacuum completely when closed. Perhaps there are certain weird conditions under which the valve could close and trap vacuum in that section of the vacuum circuit. Could be quite a few obscure reasons for it.

But if both vacuum switches have it, then it's there for a reason.