Audi Workshop Service and Repair Manuals > A3 Mk2 > Brake system | Brakes, hydraulics, regulator, servo | Checking brake servo vacuum system

Checking for leaks

Note

t	The mean atmospheric pressure at sea level is 1013 mbar and decreases rapidly with increasing altitude (approx. 100 mbar per 1,000 m of altitude). Fluctuations at different locations and times also influence vacuum generation.

t	A cold engine, an active air-conditioning system as well as engine idling have an adverse influence on vacuum generation.

–	Before starting work, check all vacuum hoses for damage (e.g. cracks or animal bites) and make sure they are correctly and securely attached.

–	Connect vacuum gauge for brake servo -VAS 6721- → Chapter.

–	Open shut-off valve -A-.

–	Close shut-off valves -B+C-.

–	Start warm engine (>60° C), press accelerator briefly (engine speed greater than 2000/min).

Under normal conditions (see notes), the generated vacuum should be between 600 and 950 mbar (depending on engine version).

–	Open shut-off valve -C- to evacuate brake servo.

–	Switch off engine.

–	Read and make a note of measured value shown.

A vacuum decrease of 400 mbar in 12 hours is permissible.

If the vacuum decrease is greater, check for leaks in the vicinity of...

1 -	Brake servo

2 -	non-return valve, vacuum hoses with connectors or vacuum pump/intake manifold.

If there are large leaks, the vacuum will decrease rapidly within a few seconds.

Vacuum check in vicinity of brake servo:

–	After generating vacuum, close shut-off valve -A- in order to check brake servo vacuum system.

Vacuum check in vicinity of non-return valve, vacuum hoses with connectors and vacuum pump/intake manifold:

–	After generating vacuum, close shut-off valve -C- in order to check vacuum system from vacuum gauge for brake servo -VAS 6721- to intake manifold or vacuum pump.

Open shut-off valve -B- to facilitate removal of hose connections and adapter.

Audi Workshop Service and Repair Manuals