The clutch master cylinder is directly connected to the clutch pedal and serves as the portion of the hydraulic system that generates hydraulic pressure. Master cylinders are divided into two primary categories based on the internal mechanism used to generate hydraulic pressure: seal over port systems and center-feed type systems.
The seal over port system uses a push rod attached to the clutch pedal to produce piston movement in the master cylinder. When the piston moves, seals attached to the piston move forward.
The primary seal passes over a small port in the wall of the cylinder, which allows fluid flow to and from the reservoir. Once the primary seal passes over the port, the fixed portion of fluid contained between the master and slave cylinder is moved toward the slave cylinder. This results in disengagement of the clutch. One variation of the seal over port is a plunger type master cylinder. Rather than using the attached seals that move with the pistons, the piston instead moves through the seals.
The center-feed type system uses small hooks attached to the nose of the piston. When the clutch pedal is not depressed, these hooks engage a flange on the valve stem. A circular rubber seal on the opposite end of the valve stem is held away from the master cylinder body. This allows fluid flow to and from the reservoir. When the clutch pedal is depressed, the hooks no longer hold the valve stem and it is pushed to the bottom of the master cylinder body.
The reservoir is now separated from the fixed fluid, contained between the master and slave cylinder. As the clutch pedal is further depressed and the master cylinder piston moves even further, fluid is forced toward the slave cylinder, resulting in the disengagement of the clutch.
Master cylinders are also subdivided into two additional categories: Integral reservoir systems and remote reservoir systems. The fluid reservoir of an integral reservoir system is part of the master cylinder body. The fluid reservoir of a remote reservoir system is attached to the master cylinder body via a rubber hose. The use of integral or remote reservoirs is dictated by the available room in the engine compartment. Both integral and remote reservoir types are used in seal over port and center feed type systems.