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The power bleeder hooks up to the fluid reservoir. The bleeder has a jug into which you put brake fluid. There is a pump which pressurizes the jug (to 10-15psi)... which in turn pressurizes the hydraulic system. When you open the bleed valve on the clutch slave (or brake caliper), the pressure forces the air and old fluid out, filling the lines with fresh fluid from the power bleeder. When bleeding the clutch, I pumped the master cylinder a couple times to knock any bubbles loose.