Check the vacuum hose

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2007 Volvo V70, 2006 Volvo V70, 2005 Volvo V70, 2004 Volvo V70, and 2003 Volvo V70. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Vacuum Hose Connections At Manifold And Check Valve

Courtesy of VOLVO CARS OF NORTH AMERICA.

Check:

the vacuum hose connections at the manifold and the check valve
that the vacuum hose is not crimped or rubbing anywhere
that the vacuum hose inner weave has not broken and blocked the through flow.
Disconnect the hose. Start the engine. Check the vacuum.