Abstract: An automatic safety brake system for use with the air brake system of a tank-type vehicle for transporting and delivering volatile liquids. The automatic safety brake system has a connector to which hoses are connected for delivering liquid to or from the vehicle tank. A port is formed in the connector and has one opening which is connected by a hose to the emergency air line of the vehicle air brake system. Another opening of the port is adapted to contact in sealing engagement with a removable sealing cap, the cap also covering the opening of the connector through which the liquid flows to and from the vehicle tank. Tightening of the sealing cap on the connector seals the port allowing pressure to build up in the emergency line of the vehicle air brake system. Loosening of the cap opens the port to atmosphere thereby exhausting the emergency line of the vehicle air brake system and automatically applying vehicle brakes.