Actually, although this has been printed in many "tech" manuals, the sight glass is a VERY poor way of charging. This is because under certain conditions -- causing high head pressure -- charging to a "no bubbles" condition can result in a HORRID "overcharge."
Charge really should be weighed in or charge by superheat or subcooling or a combination of both
Vehicles are particularly difficult because of the problem of getting enough air through the condenser when testing.
