WARNING: This page is about a different car, the 2006 Audi A4, 2005 Audi A4, 2004 Audi A4, 2003 Audi A4, and 2002 Audi A4. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Checking Pressure Relief Valve In Filler Cap
Courtesy of VOLKSWAGEN UNITED STATES, INC.
Using VAG1274/9 adapter attach VAG1274 cooling system tester to coolant reservoir tank cap.
Pump using hand pump.
Pressure release valve must open at a positive pressure of 1.4 to 1.6 bar