Abstract: A vacuum breaker for use on a moving tank for allowing atmospheric air to enter the tank to relieve a reduced pressure condition in the tank. The vacuum breaker has a light-weight closure assembly including a closure member of relatively large area. The closure assembly is normally held closed by permanent magnets. The magnets and the light-weight construction of the closure assembly substantially prevent the vacuum breaker from opening due to vibration of the moving tank. However, the vacuum breaker opens readily in response to a relatively slight reduced pressure in the tank because of the large area of the closure member. The permanent magnets are shielded from undesirable proximity of ferrous objects to prevent the magnets from being drained. The vacuum breaker may be opened manually to test for a positive pressure condition in the tank.