An assembly (12) for testing an evaporative emission control system (10) of an automotive vehicle which controls emission of volatile fuel vapors. The assembly is used to determine if a leak is present in a portion of the system which includes a vapor collection canister (14). The leak detection assembly incorporates a vacuum actuated canister vent control valve (32) which seals the evaporative system so the leak detection test can be performed. A vacuum actuated pump (28) performs a leak detection test. An engine control (26) initialize the pump by drawing air into a pump cavity and also closes the vent control valve (32). After the initialization period, the pump is activated to pressurize the evaporative emission control system. Once a control pressure is achieved, the cycle rate starts to drop off. If no leak, the pump will continue to pump at a rate that will be representative of the flow characteristics of the size of the leak. After the test has been concluded, the vent control valve (32) will unseal the system after the vacuum has been bled out through the orifice/check valve (31) that has been holding the seal during the test.