I have been curious about this as well. I looked and sure enough there is a plug on the front end of my Hydrovac unit. Here is a picture, the plug has a square head. The diagram in my 1955 shop manual does not show this plug, but it gives a clear description of the operation. Focusing on this plug, a simple description:The power cylinder has vacuum in it on both sides of the power cylinder piston when the engine is running and the brake pedal is not being pressed. When the brake pedal is pressed, atmospheric pressure is applied to the front end of the power cylinder. So from this the plug needs to be air tight. I've never added oil, I will now.
