a/c valve core

The screw-on cap can stop the the leak if you put a drop of oil on its seal.

standard valve core. I find on the web some "high

Might want to just tighten it and/or the cap to stop the leak. Dishwashing liquid works good to check for leaks but don't get it in the port.

Or you might want to swing by a shop. They can replace the core under pressure with a special tool.

Here is a catalog page of cores:

Even better, get a *good* brass cap that has an o ring.. That will stop it for sure if it's tight. It's possible the valve core could be changed out, but not all can. Some have "stops" so to speak to keep the valve from unscrewing all the way. You would have to force it, and I don't like doing that too much. I'd rather just use a sealing cap if tightening the valve doesn't help. MK