A vacuum power booster with automatic waste liquid discharge for a water vacuum extraction apparatus. A first vacuum source is coupled to a first waste liquid storage tank in a main housing, typically vehicle mounted. A second, auxilliary vacuum source is mounted in a mobile housing remotely located from the first housing which contains a second auxilliary waste liquid storage tank. A vacuum pickup implement is coupled to the mobile waste liquid storage tank for extracting and transmitting dirty liquid from the surface being cleaned to the second waste liquid storage tank. The liquid in the second waste liquid storage tank is automatically discharged and transmitted to the first waste liquid storage tank when the level of such liquid reaches a predetermined height or level within the second waste liquid storage tank. An outlet formed at a predetermined height above the bottom of the mobile housing in communication with the second waste liquid storage tank is connected by a conduit to the first waste liquid storage tank to discharge liquid to the first waste liquid storage tank when such liquid level reaches the outlet. Alternately, a valve is interposed between the outlet in the mobile housing and a conduit connecting the mobile housing to the first waste liquid storage tank for selectively opening and closing the fluid flow connection between the first and second waste liquid storage tanks depending upon the level of waste liquid within the second waste liquid storage as detected by a liquid level detector.