How Master Cylinders And Slave Cylinders Work And Their Importance

Hidden behind the pedal box and nestled deep within the hydraulic systems of a car, the master and slave cylinders provide assistance to potentially the most important systems of a car

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How Master Cylinders And Slave Cylinders Work And Their Importance - General Mechanical Advice

Every ounce of physics happening within a car is based around energy transfer. Energy can start in one form and be forced to change to another; chemical to heat, heat to sound, potential to kinetic. And there are multiple components that go about amplifying certain inputs to useful outputs, many of them through hydraulics. Although many driver inputs are slowly working their way towards electrification or ‘by wire’, plenty of cars still use the old-fashioned hydraulic and cable methods that we all know and love.

The master and slave cylinders are used as focal points for pressure to be created; so what are they and how do they work?

How Master Cylinders And Slave Cylinders Work And Their Importance - General Mechanical Advice

The clutch and brakes on a car need a little help to actuate, considering the forces that they are designed to oppose, therefore hydraulic pressure is created within the master and slave cylinders to provide the required force. The master cylinder can be found directly behind the pedal box, connected to a respective pedal.

Let’s start with the brakes; as force is applied through the brake pedal, a pushrod is pushed through hydraulic fluid to create hydraulic pressure. Therefore a non-hydraulic pressure (from the pedal) is converted into a hydraulic pressure through energy transfer. The master cylinder features a reservoir of fluid to store the amount needed and below this reservoir sits the bore in which is housed two pistons separated by a spring. As the pistons are acted upon via the pushrod, they push through the hydraulic fluid, compressing against it and creating an internal pressure. This pressure is then sent through the brake lines to actuate either a brake drum or caliper.

The brake master cylinder can be found at the back of this diagram, with the brake servo in front
The brake master cylinder can be found at the back of this diagram, with the brake servo in front

Each piston corresponds to a different brake ‘circuit’ which doubles up as an added safety measure. One piston is used to send hydraulic pressure to the front right and back left brake callipers, and the second is used to assist the front left and back right respectively. Using this method – if the first piston was to fail or primary hydraulic pressure is lost, the pushrod would simply need to be pushed further down the bore until the failed piston met up with the second piston, thus allowing at least some hydraulic pressure to be applied. This means that at least some braking force is applied to both the front and back wheels of the vehicle, with the reservoir also being split in two to aid the division of hydraulic pressure.

The master cylinder effectively works as a hydraulic pump, from which fluid is fed to the slave cylinders further down the line. The slave cylinder is found at the other end of the hydraulic system and works in the opposite way to the master cylinder. Once the hydraulic fluid has transferred through to the slave cylinder, the pressure is used to actuate a linkage back and forward, converting the movement of fluid back into mechanical movement of the linkage.

The braking system showing the route of the hydraulics
The braking system showing the route of the hydraulics

The slave cylinders in a car are therefore used to finish the amplification of the forces from your foot inputs through to the clutch and brakes respectively. In the case of the clutch, the slave cylinder actuates the clutch fork to disengage the clutch friction plate from the flywheel, with a return spring reversing the process. And the slave cylinder found at each set of brake calipers on a car is used to close the brake pads around the brake disc. To add assistance to the braking system, a brake servo sits in front of the master cylinder and uses the vacuum created within the inlet manifold to further amplify the hydraulic pressure within the braking system.

Video via YouTube channel Educational Mechanics

Master and slave cylinder failure can be a bit of a nightmare, with most faults coming from leaks in hydraulic fluid leading to a lack of pressure. This can lead to a gear change becoming incredibly difficult to perform as the force being applied through the clutch pedal is simply not enough to efficiently actuate the clutch fork. Therefore a faulty slave cylinder can render a car nearly undriveable if failure occurs. Obviously a hydraulic fault like this within the braking system can be even more catastrophic, which is why the master cylinders are engineered with a certain safety factor using the secondary piston within the mechanism.

The brake slave cylinder or 'wheel cylinder' on a rear drum brake
The brake slave cylinder or 'wheel cylinder' on a rear drum brake

If you ever feel like your clutch has ‘gone’ due to an extremely stiff gear change which may even result in a grinding of gears, the chances are it’s probably a hydraulic failure from either the master or slave cylinder. Possibly it’s just a small hydraulic leak that needs to be plugged followed by a quick reservoir top-up, or it could be genuine damage to the piston or spring mechanisms within the cylinders. So next time you press down on either the brake or clutch pedals, you’ll now know exactly what’s going down!