A vacuum breaker should not leak at all. The purpose of the vacuum breaker is to control the atmospheric pressure inside a geyser. If it is leaking, it means the rubber seal inside has either broken or has perished and needs to be replaced.

Some faucets have a built in vacuum breaker that can not be removed. Many have a srew on type, it usually has some hex screws on the side to keep it from being removed. Loosen the hex screws, and the vacuum breaker should screw off. Righty tighty lefty loosey.

To demonstrate atmospheric pressure.

An atmospheric condenser operates naturally at atmospheric pressure (1.013bar).

A vacuum condenser operates at pressures below atmospheric and will use some sort of pump to provide a vacuum.

That would depend on the application as there are many uses and shapes and specifications for a vacuum breaker