To enable this, the hydraulic system must be filled with a suitable, non-compressible “brake fluid”. All brake fluids are referred to as “DOT brake fluids” (DOT is an abbreviation for “Department of Transportation”).
The hydraulic system must be entirely free of gas-bubbles, as these would be compressible and would reduce the brakes’ efficiency. To achieve this, the hydraulic system is pre-evacuated with a vacuum prior to being charged with the DOT fluid. During the charging stage, some droplets typically do enter the vacuum line and subsequently the vacuum pump. 
Unfortunately, most DOT liquids are harsh; they can degrade a vacuum pump’s oil and seals. To handle this, Leybold has developed special “DOT pumps” which use adapted sealing materials and even use DOT fluid as pump oil. These special versions are proven components of most automotive production lines around the world.