If your float valve inside your carburetor float bowl works, you should have no leaks, regardless of your petcock condition.
If you have a leak, recommend you inspect the float valve point and its seat. Point should be smooth and pliable (not hard); seat should be clean and smooth.
Commend to you the thread, "Carb Overhaul," on this website.
Anyhow, yes, you can use a manual-activation only petcock, and yes, if you do, block off the carburetor vacuum port.
Last edited by Damocles; 07-28-2017 at 08:49 PM.