Disconnect the brake booster vacuum hose (A) at the booster. The check valve (B) is built into the hose.

NOTE: If the check valve is faulty, replace the brake booster vacuum hose/check valve as an assembly.

Start the engine, and let it idle. There should be vacuum available. If no vacuum is available, the check valve is not working properly. Replace the brake booster vacuum hose and check valve, and retest.