Between intake manifold and vacuum pump

WARNING: This page is about a different car, the 2017 Mazda 3. However, it is still accessible from the selected car via links, so may be relevant.

Remove the vacuum hose (between intake manifold and vacuum pump). (See INTAKE-AIR SYSTEM REMOVAL/INSTALLATION .)
Blow air into the vacuum hose (between intake manifold and vacuum pump) using your mouth from the vacuum pump side and verify that the air flows to the intake manifold side, then blow air from the intake manifold side and verify that the air does not flow to the vacuum pump side.

Fig 1: Checking Check Valve Between Intake Manifold And Vacuum Pump Side

Courtesy of MAZDA MOTORS CORP.

If there is any malfunction in the check valve (between intake manifold and vacuum pump), replace it with the vacuum hose (between intake manifold and vacuum pump) as a single unit. (See INTAKE-AIR SYSTEM REMOVAL/INSTALLATION .)