A clutch master cylinder contains a reservoir that stores the brake fluid. It is connected to the clutch slave cylinder through hoses. When you push the clutch pedal, brake fluid flows from the clutch master cylinder to the slave cylinder, applying the pressure necessary to move (engage) the clutch. It has internal and external seals that can fail. If the internal seals wear out, the clutch fluid will continue to circulate inside instead of being directed to the slave cylinder. If the external seals wear out, the clutch master cylinder will leak brake fluid, reducing the amount of fluid in the car and causing the clutch to malfunction. You may also find leaked brake fluid down by the clutch pedal (inside the car). In that case, the clutch pedal will go all the way to the floor when you apply the clutch.
Inspect clutch master cylinder and slave cylinder. Check the condition of brake fluid. Inspect the operation of the clutch pedal. Remove and replace the clutch master cylinder and slave cylinder. Bleed all air from the master cylinder. Test drive the vehicle.
The clutch master cylinder stores brake fluid. When the clutch pedal is depressed, the fluid is transferred from the clutch master cylinder to the slave cylinder, which provides the pressure needed to engage the clutch. If the clutch master cylinder is leaking, it won’t hold the proper amount of brake fluid, and there won’t be enough pressure from the fluid to engage the clutch.