The hydraulic throw out bearing takes the place of the slave cylinder. Do your research, they're not as simple as they sound, getting them set up right (adjusted) during installation is critical. Depending on the age and condition of your clutch disk, pressure plate, and flywheel you might want to think about replacing them, too, while you have the transmission out. I'm assuming the reservoir you're referring to has a clutch master cylinder attached to it. Some have a clutch master cylinder under the dash with a remote reservoir.