I can't seem to stop my anti-freeze sill cock valve from leaking even with new gasket. The stem doesn't seem long enough for it to seat. Is there an adjustment I am overlooking/

Can you post a photo of the sill cock ?

In the interim: make sure you got the right parts for your faucet. Best thing to do is to close water, remove the leaking stem and take it with you to the store. That way you are sure you are buying exactly the same replacement part.

Back to you. Milo