BRB-83-911SC

Can a leak in the HVAC vacuum system affect the engine control vacuum system? I assume that it can only if the blue check valve by the brake booster has also failed. True or False?

westija

I would venture to say no, as I believe the engine produces vacuum for the A/C and that can be done continuously, but others will likely confirm or correct me

Tails

I can only speak for the S4 models, without investigation.

The vacuum from the booster evacuates the cruise control vacuum tank and operates the vacuum diaphragms of the HVAC. In addition it also operates the resonance valve on the intake manifold and is opened via an electric solenoid valve. If you have a large vacuum diaphragm leak the resonance valve may fail to actuate on start up due to inadequate vacuum IIRC.
With the MY83 I'm not sure, you need to check where the vacuum line from the intake manifold via the rubber spider is connected to, to see if there is an effect on the engine.