Between intake manifold and vacuum pump

Remove the plug hole plate. (See PLUG HOLE PLATE REMOVAL/INSTALLATION [SKYACTIV-G 2.0, SKYACTIV-G 2.5] .)
Remove the vacuum hose. (See VACUUM HOSE REMOVAL/INSTALLATION .)
Blow air into the vacuum hose from the vacuum pump side using your mouth 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: Blowing Air Into Vacuum Hose From Vacuum Pump Side

Courtesy of MAZDA MOTORS CORP.

If there is any malfunction of the inner check valve, replace it together with the vacuum hose as a single unit. (See VACUUM HOSE REMOVAL/INSTALLATION .)