What is Master Cylinder? Definition, Types, Parts or Construction [Notes & PDF]

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In automobiles, the master cylinder is a control device that converts force on the brake pedal into hydraulic pressure. It controls the slave cylinder located at the other end of the hydraulic brake system.

The master cylinder is a component in the car’s braking system that drives the pressure exerted on the brake pedal to the braking mechanism at the vehicle’s wheels.

By definition, a Master cylinder is a cylinder or water-powered gadget that uses pistons and cylinders that are arranged in such a way that the mechanical power exerted on the brake pedal by the driver of the vehicle or the brake switch in bicycles is converted to a water-driven load. The load is thus used for braking by the brake caliper.

In a hydraulic braking system, the chamber is a component that delivers the required weight or braking power to the final braking segment after the conversion of input mechanical power by the driver on the brake pedal or brake switch.

It works on the basic principle of Pascal’s law where a high-pressure fluid is obtained at a smaller outlet area by application of force at the larger inlet area.

The master cylinder is an equally essential component of a disc brake assembly, just like the brake disc/rotor. It is generally made up of aluminum or cast iron.

It works like a throttle system. As the throttle supplies fuel to the engine, the master cylinder supplies brake fluid to the caliper assembly at high pressure. It provides the required amount of braking force to the final braking components after the multiplication of the mechanical force applied by the driver on the brake pedal or brake lever.

This system uses the property of the brake fluid to work at high temperatures and pressure. The ideal properties of brake fluid are high operating temperatures, viscosity-temperature properties, physical and chemical stability, protection of metals from corrosion, and lubrication effect.

History of Master Cylinder

The invention of the master cylinder dates back to 1918. Malcolm Lougheed developed a system that used liquid pressure with cylinders and tubes. It was a huge achievement since it required less force than the mechanical brakes to stop the car.

The problem with Lougheed’s brakes is that they malfunctioned due to leakage. Chrysler took and made some improvements to Lougheed’s original system. The system got renamed as Chrysler-Lockheed hydraulic brakes and was used in Chrysler vehicles from 1924 to 1962.

Other auto manufacturers had converted to hydraulic systems as well by the 1940s. Lougheed had got the concept right, but his design was flawed. Also, since it was a single master cylinder system, all of the brake lines were interconnected.

So, if there was a leak or a faulty component in the system, the braking system would stop working. To overcome this, Wagner Electric invented a dual-cylinder brake system in 1960.

This system consisted of a dual master cylinder separating the front and rear hydraulic lines. So, in case one line developed a leak, the other line would still work, and the braking system won’t completely stop.

Types of Master Cylinder

Open System

An open system master cylinder assembly contains a bladder inside the reservoir tanks. These bladders help in adjusting the level of brake fluid in the master’s cylinders. One side of the bladder is exposed to the environment, therefore it expands or contracts according to changes in heat.

The advantage of an open system is that automatic adjustment of brake fluid level in the master cylinder assembly takes place. Protections of the disc brake system from overheating. Since the level of brake fluid is automatically adjusted by the expansion or contraction of the bladder, it does not affect the performance of braking.

The disadvantage of an open system is that it adjusts automatically. When the brake pads malfunction, the open system supplies an excess amount of fluid to the caliper bore so that the piston adjusts according to the malfunctioning pad.

This condition could damage the disc and caliper assembly. It also creates noise and affects the performance of the brakes. Under filled bladder under vacuum and an overfilled bladder under pressure can affect the braking performance. As a result, it is considered old technology.

Closed System

The closed system master cylinder does not have a bladder in a reservoir tank. Therefore the system needs manual adjustment.

The advantage of such a system is that the brake fluid level can be checked through the reservoir tank and changed if it is dark or does not have recommended levels. If the brake pads are worn out, they can be easily detected and changed in such systems.

The disadvantage of such a system is it can cause overheating problems. If the level of brake fluid is not at the optimum level, it can affect the performance of braking. If an air bubble forms anywhere throughout the system, it can affect braking performance. Bleeding of the brake fluid is required to rectify the formation of air bubbles when changing the brake fluid.

Single Cylinder

Single-cylinder is simple in construction type master cylinder and is intrinsically similar to a plastic medical syringe. The brake pedal pushes the pushrod and pistons inside the cylinders, which flows fluid through the lines and into the slave cylinder. Springs inside the cylinder push the piston back to its original position when the brake pedal is released.

The negative pressure created due to vacuum draws the brake fluid from the lines and the brake fluid from the reservoir into the cylinder. Automakers switched to more redundant tandem master cylinders long ago, but many race car manufacturers prefer to use a pair of singles cylinders instead of a single tandem cylinder to control brake pressure bias at the front and rear end.

Ported Tandem Cylinder

A tandem cylinder consists of two pistons in a single cylinder. The primary piston is connected to the brake pedal and is further connected to the secondary piston.

When the brake pedal is pressed, the primary piston exerts pressure on a spring attached to the rear end of the secondary piston. Once that spring is fully compressed, the secondary piston begins to push the fluid through to pressurize the braking system.

The reservoir inlet port allows the brake fluid to flow past the primary piston to maintain even pressure on both sides. When the brake pedal is released, the pressure of the spring pushes the piston back and a compensation port from the brake fluid reservoir introduces excess fluid into the compression chamber.

The compensating port is necessary to accelerate the brake release process, which would otherwise be hindered by the movement of fluid flowing backward through the brake lines.

Portless Master Cylinder

Portless Master Cylinders offer a faster brake release mechanism than the standard designs that use a compensating port. The Portless Master cylinder uses a valve assembly in the piston that opens up to equalize pressure when the brake is released.

This phenomenon allows the compression cylinders to do without a compensating port, which is more restricted to fluid flow and reduces pressure from the brake system under the initial application.

Since the pointless cylinder is fast-reacting, it works better with the anti-lock braking (ABS) system, which uses rapid pressure modulation to adjust the braking force.

Parts or Construction of Master Cylinder

The master cylinder consists of various parts such as a pedal, reservoir, cylinder, pistons, return spring, valves, hose pipes, fluid check valve, and seals. These are further explained below:

Pedal:

This is the only part that can be controlled by the driver of the automobile. The driver uses the pedal to slow down or stop the vehicle according to the need.

The force exerted on the pedal by the driver has a direct effect on the master cylinder assembly. The brake pedal or lever is directly in contact with the master cylinder assembly.

Reservoir:

The reservoir is a storage that is used for storing the brake fluid. It is usually made up of plastic or fiber. The reservoir is connected to the cylinder via the inlet valve and supplies brake fluid into the system whenever required.

Brake fluids are viscous fluids used in hydraulic braking systems. The Brake fluids convert the force exerted on the brake pedal into pressure. They operate at high temperatures and pressures. These are graded just like the engine oil. The DOT series 1,2,3,4,5,5.1 are examples of the brake fluids used.

DOT stands for the department of transportation which has set standards for brake fluids. The DOT number is used to indicate the boiling point of the fluid. A higher DOT number indicates better quality and greater ability to withstand high temperature and performance environments.

DOT 3, 4, and 5.1 are glycol-based fluids while DOT 5 is silicone-based fluid. Glycol-based fluids are typically used in performance-based cars with ABS( anti-lock braking system). These are generally rectangular or square-shaped.

Cylinder:

The cylinder in the master cylinder assembly is an air-tight housing inside which the piston moves. It is the compression chamber in which the movement of the piston causes the brake fluid to be compressed at high pressure.

It is caused due to the moment of the brake pedal which in turn causes conversion and multiplication of force. It is usually made up of cast iron or aluminum.

The cylinder is connected to the reservoir through the inlet valve and brake liners through the outlet valve. The cylinder is an important component as it maintains high air pressure inside to keep the system operating without losses.

Single circuits consist of a single compression chamber and dual circuits have two compression cylinders with inlet and outlet valves for each.

Piston Assembly:

The piston assembly consists of a push rod and piston. The push rod is the part connected with the brake pedal and the piston.

When force is exerted on the brake pedal, it moves the pushrod to push against the piston. The piston is the reciprocating part inside the master cylinder that moves inside the cylinder when force is exerted by the movement of the brake pedal.

The piston is connected to a push rod which causes compression of the brake fluid inside the cylinder and generates higher hydraulic pressure on the brake calipers.

In a single circuit, only 1 piston is used and double circuits use 2 pistons (primary and secondary piston). The primary piston is attached to the pushrod and the secondary piston is attached to the returning spring of the primary piston.

To prevent leakage of the brake fluid, rubber seals are present on either side of the piston. A rubber boot is used to cover the pushrod end of the cylinder to prevent the dirt from entering inside the brake lines.

Return Spring:

It is a simple coil-type spring that is used in drum and disc brakes. It helps the piston to return to its original position after the brake pedal is released. In a dual circuit system, two springs are used namely a primary spring and a secondary spring.

Valve:

It is the outlet valve that attaches the cylinder to the brake lines. The compressed brake fluid is transferred to the brake caliper through this valve. In a single circuit master cylinder, there is only one valve and a double circuit master cylinder consists of two valves (primary and secondary).

Fluid Check Valve:

It is also called the residual check valve. It is used to keep a certain amount of pressure inside the brake fluid line. This helps in the quick application of the brakes. Due to the fluid check valve, the brake pedal does not need to be pressed longer.

Seal/Cup:

The gap between the cylinder and piston is covered with the help of seals. The primary seal is attached at the spring side end of the piston and the secondary seal is attached on the opposite side.

The gap between the primary and secondary seal is filled with brake fluid by the inlet valve. The secondary seal prevents leakage of fluid behind the piston. A rubber boot is also used to prevent the entry of debris/contaminants into the master cylinder assembly.

How does Master Cylinder work?

Based on the number of cylinders used, the master cylinder can be classified into two sections:

Single Circuit:

In a single circuit master cylinder, when the brake pedal is in non actuated position, the piston remains at its original position which keeps the inlet valve of the reservoir closed due to which there is no incoming brake fluid from the reservoir to the compression chamber.

When the brake pedal is in the actuated position, the piston which is connected to the brake pedal through the connecting rod moves. This in turn opens the inlet valve due to which incoming brake fluid from the reservoir to the compression chamber takes place.

The brake fluid is compressed inside the compression chamber due to the movement of the piston inside the cylinder. The movement of the piston decreases the surface area for the brake fluid, thus increasing the pressure on the fluid.

After compression up to a certain pressure, the outlet valve opens and the compressed brake fluid is transferred to the brake caliper through the brake lines for further brake actuation.

Once the brake pedal is released, the wheel brakes are released and the pressure in the braking circuit falls. The piston returns to its neutral position which creates a vacuum, which opens the central valve and therefore ensures that brake fluid is replenished.

After the release procedure is complete, the non-actuated position is achieved and it opens the compensation valve which transfers the excess brake fluid in brake lines to the reservoir.

Double Circuit:

The working of the Double circuit master cylinder is almost similar to that of the single-cylinder master cylinder. The difference is the presence of two separate independent circuits of braking in the double circuit.

When the brake pedal is in non actuated position, the piston remains at its original place, closing the inlet valve of both the compression chambers, which in turn cuts the incoming brake fluid from the reservoir to the compression chamber.

When the brake pedal is actuated, the primary piston moves due to which opening of the primary inlet valve takes place.

Due to the movement of the primary piston, compression of the brake fluid inside the primary chamber takes place.

After completion of the compression in the primary chamber, the primary outlet valve opens up and the compressed brake fluid is further sent to brake calipers through brake lines and actuation of the primary circuit brakes takes place.

Once the primary piston movement is complete i.e. at its extreme end, the secondary piston starts moving because of the force applied by the primary piston’s spring which in turn opens the secondary valve.

The incoming of brake fluid from the secondary reservoir to the second compression chamber takes place.

The brake fluid from the secondary reservoir is then compressed and after complete compression, the secondary outlet opens up and this highly compressed fluid is sent to the brake calipers through brake lines and actuation of the secondary circuit brakes take place.

The returning mechanism of the piston after releasing the brakes in a dual cylinder is the same as that of a single cylinder.

After the release procedure is complete, the non-actuated position is achieved and it opens the compensation valve which transfers the excess brake fluid in brake lines to the reservoir.

Master Cylinder Function:

#1. Pumping brake fluid

Works on the principle of Pascal’s law, providing high-pressure fluid at a smaller outlet area by application of force at a larger inlet area.

#2. Equal braking effect

The Master cylinder applies hydraulic power to the brakes. It works in a similar manner as that of a syringe.

Provides a safe braking system Most cylinders use two sets of compression chambers that tolerate independently. It is done to ensure brake safety in case of a broken brake line or other issues. Even if one brake set fails, the other set will still have powered brakes.

Stores excess brake fluid Master cylinder stores excess brake fluid in the reserve so that when the pressure is applied the brake fluid is flushed out of the cylinder into the brakes.

#3. Recollects brake fluid

The master cylinder recollects the amount of brake fluid in the reservoir after releasing the brake.

#4. Compensates for fluid volume changes

The reservoir compensates fluid for any changes in the fluid volume in the pipeline due to variations in temperature and in case of any leakage.

#5. Prevent entry of air

The system is completely sealed to prevent the entry of air into the system with the help of seals and cups.

Advantages of Master Cylinder:

The hydraulic braking in the master cylinder requires less effort from the driver for braking and the brake pedal does not need to be pressed for a long time for braking.
The instantaneous movement of brake fluids inside the cylinder provides a quick braking effect on the wheels of the vehicle.
In a dual cylinder arrangement, even if one brake line fails the other brake line will provide enough braking power.
The braking pressure on each wheel is equal and does not have a pressure bias.
The use of brake fluid overcomes the excess heat issue inside the braking system.

Disadvantages of Master Cylinder:

A leakage in the cylinder or pipeline can affect the performance of the master cylinder braking system.

Contaminants or residues in the brake fluid can affect the performance of the system.
Below-par levels of brake fluid levels can damage the parts of the system.
Periodic change of brake fluid is necessary for optimum working.
Bleeding of the brake fluid is necessary while filling to avoid the formation of air bubbles in the pipeline.
The system will stop working if the hydraulic pressure inside the cylinder is lost.
If the cylinder malfunctions, the brake will behave abnormally and pressure will not get distributed properly.

Applications of Master Cylinder:

Single circuit

It is mostly used in two-wheelers namely Bajaj, TVS, Honda, and so on.
It is also used in other lightweight vehicles like rickshaws.

Dual circuit

It is used in almost every vehicle with a hydraulic braking system.
Dual circuit braking is a mandatory standard in many countries as a safety measure to prevent accidents.

More Resources:

References:

What is Master Cylinder?

A Master cylinder is a hydraulic braking system used in automobiles in which a cylinder and pistons are arranged in such a way that the mechanical force exerted by the driver of the vehicle either by brake pedal or by brake lever is converted into hydraulic pressure by feeding brake fluid into the braking circuit which is then transferred to the brake caliper for actuation of the brakes.