Between vacuum pump and power brake unit

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

Fig 1: Checking Check Valve Between Vacuum Pump And Power Brake Unit

Courtesy of MAZDA MOTORS CORP.

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