Vacuum in fuel tank

[Belairbob]

I have a 72 with an earlier year tank. The charcoal canister and lines were removed.
I have a vented cap but also hear vacuum when I remove the cap for fill up. The
Cap has the small hole in it. You can see the check valve through the hole. You can push the check valve in. Wouldn't this be to relieve preasure instead of letting air in? Thanks, bob

[LockDoc]

Originally Posted by Belairbob

I have a 72 with an earlier year tank. The charcoal canister and lines were removed.
I have a vented cap but also hear vacuum when I remove the cap for fill up. The
Cap has the small hole in it. You can see the check valve through the hole. You can push the check valve in. Wouldn't this be to relieve preasure instead of letting air in? Thanks, bob

I would get a new cap. I always buy one for a '69 or earlier truck just to make sure.... Yours is not vented or it would not hold a vacuum in the tank.

LockDoc

[Belairbob]

Thanks for answering LockDoc!

I have a new one that is also suppose to be for the 69-70 but it looks
the same. I tried blowing into the check valve hole and sucking air back -
I can push air through this way as well as pull air back.
I'm thinking the rubber gasket must be sealing the lid to the neck
and cutting off the air flow, or something like that.

Bob

[Lattimer]

When well sealed, mine will build pressure in the tank, so I get a whoosh sound when I open it. The check valve lets air in, but not out if I blow through it.

I have another cap that lets air both ways and leaks gas like crazy when I turn right.