Fits FJ40/45, FJ55, BJ & HJ Series – 1/1975 – 7/1980 The clutch master cylinder is a device that transforms mechanical force into hydraulic pressure. As the driver presses the clutch pedal, the pedal lever applies force to the clutch master cylinder which transmits hydraulic pressure to the clutch release (slave) cylinder that disconnects engine power to the transmission. If your master cylinder is leaking, it is a sure sign that it is time to replace it, as the seals and/or cylinder wall has worn, allowing fluid to escape. Bolt Pattern: 2″