Vacuum Check Valve
WARNING: This page is about a different car, the 2004 Lexus ES 330. However, it is still accessible from the selected car via links, so may be relevant.
Working at brake booster, remove clamp and disconnect vacuum hose from vacuum check valve. Remove vacuum check valve. Check that there is ventilation from booster to engine, and no ventilation from engine to booster. See Fig 1. If any fault is found, replace vacuum check valve.