WARNING: This page is about a different car, the 2006 Audi A4, 2005 Audi A4, 2004 Audi A4, and 2003 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.
Install tester VAG1274 with adapter VAG1274/9 onto cap.
Operate hand pump.
Pressure relief valve must open at pressure of 1.4 to 1.6 bar.