Use an additional valve core removal tool in your vacuum setup to valve off and protect the vacuum gauge sensor from oil contamination when refrigerant is added into the tubing. If the sensor gets oil on it, the sensor will not perform correctly and the sensor will need to be cleaned. Often this is accomplished with rubbing alcohol but always follow manufacturers' instructions for cleaning.
CPS Vacuum Micron Gauge -http://amzn.to/2v1nM3O
1/4" Appion Valve Core Removal Tool -http://amzn.to/2uYr8WL
5/16" Appion Valve Core Removal Tool -https://amzn.to/2JHmfbP