When glass is used for both the stopcock body and the plug, the contacting surfaces between them are special ground glass surfaces (see Laboratory glassware) often with stopcock grease in between. Special glass stopcocks are made for vacuum applications, such as in use with vacuum manifolds.
The use of glass for both the stopcock body and the plug necessitates special ground glass surfaces between them, often coated with stopcock grease (see Laboratory glassware).  Special glass stopcocks are specifically made for vacuum applications, like those found in vacuum manifolds.