If there is any malfunction of the inner check valve, replace it together with the vacuum hose as a single unit.
1. Remove the vacuum hose between the power brake unit and the intake manifold using pliers.
2. Verify that air can be blown from the power brake unit side of the vacuum hose towards the intake manifold side, and that air cannot be blown in the opposite direction.
If there is any malfunction of the inner check valve, replace it together with the vacuum hose as a single unit.