Clutch Master Cylinder


Hydraulic-assisted clutches are found on many types of vehicles. Hydraulic operation of clutch linkage is similar to the way brakes operate. Liquids cannot be compressed, so they can transmit motion and increase torque. The input piston is located in the master cylinder (action cylinder) connected to the clutch pedal.

A hydraulically operated clutch.

It is located next to the brake master cylinder. The output piston is located in the reaction or actuator cylinder, commonly called a slave cylinder. It is attached to the release lever at the clutch. The two cylinders are attached hydraulically by tubing and hose, just like in the brake system. Hydraulic systems are popular on custom vehicles because adaptation of the clutch release mechanism is easy.

The only difference between a clutch master cylinder and a single brake master cylinder is that the brake master cylinder has a residual check valve if it is for drum brakes. A clutch master cylinder does not have one because the clutch would stay applied as if a foot were always resting on the pedal. This would result in failure of a standard release bearing.

Visual Inspection

On vehicles with hydraulic clutch linkage, check the clutch master cylinder reservoir fluid level. It should be approximately 1/4 inch (6.35 mm) from the top of the reservoir. If it must be refilled, use approved brake fluid. Also, since the clutch master cylinder does not consume fluid, check for leaks in the master cylinder, connecting flexible line, and slave cylinder, if the fluid is low.

Some diagnosis can be done on the basis of pedal feel: A pulsating pedal is due to something internal in the clutch. The clutch will probably have to be disassembled, but looseness or misalignment somewhere could also be the culprit.

Air in the lines can cause a spongy pedal on hydraulically released cars. bleed the system to solve this problem.

Slipping Clutch

A clutch is tested for slipping by putting it in the highest gear range (this would be fifth gear on a five-speed). Set the parking brake firmly and attempt to slip the clutch as if trying to make the vehicle move. If the clutch is slipping, engine rpm will rise. If the clutch holds, the engine will die. There are several possible reasons for a clutch to slip.

Partial Engagement

Partial engagement is a common cause of a slipping clutch. In a manually adjusted system as a clutch disc wears, free play becomes less. Because of the multiplication effect of the clutch linkage parts, a small amount of wear on the clutch disc can result in a total loss of original free travel. If the wear progresses beyond this point, it will be just as if the driver is always "riding the clutch" (pushing a small amount on the clutch pedal). Slipping is the result. This ruins the clutch disc (if it is not worn out already). When a clutch slips, temperatures inside the clutch housing can reach 500°F in just a few seconds. Burn marks on the pressure plate are one of the results.

Partial Disengagement

A cause of partial disengagement is when there is a problem with a hydraulic clutch release system. Look for a low fluid level first. Then, check for a failed master or slave cylinder. Replacement of the leaking part fixes the problem. Leaks in the hydraulic system can be in hoses or lines but usually result from age of internal rubber sealing parts.

  • A leaking clutch or brake master cylinder might be leaking into the passenger compartment. Check the carpet or floormat for evidence of leakage.
  • It is possible that there might be no evidence of leakage. This is because a master cylinder might have an internal leak. The test for an internal leak is to pump the clutch pedal and hold it to the floor while starting the engine. If the clutch slowly begins to engage, there is an internal leak.
A clutch master cylinder has two rubber cup seals that can fail. Courtesy of General Motors Corporation, Service Technology Group.