A motor vehicle hydraulic brake system fluid reservoir for use with an electronic control unit. The hydraulic brake system fluid reservoir has a housing including a reservoir interior space having at least one fluid chamber, and a device for monitoring a reservoir filling level including a switch unit, which with a contact carrier projects into a guide sleeve, which extends into a separate chamber of the reservoir interior space. A sensor element or switch element is arranged on the contact carrier and is operable by a pickup carried on the guide sleeve at a switching point, at which the switch unit generates a signal for the electronic control unit. The separate chamber is connected to the fluid chamber by way of a fluid connection. Valve means are also provided, which serves to control the fluid flow between the separate chamber and the fluid chamber.