Check the vacuum hose

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.