WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Testing Relief Valve
Courtesy of VOLVO CARS OF NORTH AMERICA.
Pump air in until the relief valve opens. This should occur between plus (+) 14 and plus (+) 30 kPa.