# Brake Master Cylinder – Function , Working , main parts and Diagram

## Brake master Cylinders :

In automotive engineering, the master cylinder is a control device that converts non-hydraulic pressure (commonly from a driver’s foot) into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic system.

As piston(s) move along the bore of the master cylinder, this movement is transferred through the hydraulic fluid, to result in a movement of the slave cylinder(s). The hydraulic pressure created by moving a piston (inside the bore of the master cylinder) toward the slave cylinder(s) compresses the fluid evenly, but by varying the comparative surface-area of the master cylinder and/or each slave cylinder, one can vary the amount of force and displacement applied to each slave cylinder, relative to the amount of force and displacement applied to the master cylinder.

Function Of Brake Master Cylinders :

Brake Master Cylinder(BMC) is nothing but a highly advanced piston and cylinder assembly.

The purpose of BMC is to build hydraulic pressure and it works on the basic principle of Pascal’s Law

So how does a BMC build hydraulic pressure, it does using some complicated piston, seals and spring set-up.

Diagram Of Master Cylinder :

The principle of working is that of Pascal’s Law where a high pressure fluid is obtained at the outlet of smaller area, of a cylinder by application of force on the larger area inlet area.

The below picture is a cut section of a BMC. The main parts we will deal with and which is sufficient for understanding the function of the BMC are

1) Reservoir  which holds the brake fluid

2) Primary & Secondary Piston – which acts as piston

3) Seals – seals the port and also seals the chambers.

Master cylinder can be rightly named as heart of the hydraulic braking system. There are two main chambers viz. the fluid reservoir & compression chamber in which the piston operates(Fig. The fluid in the reservoir compensates for any change In the fluid volume in the pipelines due to temperature variations & to some extent due to leakage. To prevent leakage there are rubber seals on both ends of the piston in the compression chamber. The reduced diameter region of the piston is always surrounded by the fluid. A rubber boot covers the push rod end of the master cylinder to prevent the dirt from entering inside towards the brake lines side of the compression chamber, there is a fluid check value with a rubber cup inside. It serves to retain the residual pressure in the brake lines even when the brakes are released.

Working of Master Cylinders :
When the brake pedal pressed the push rod moves toward left by moving the piston against the spring force as it covers bypass port, a pressure built up in the compression chamber when sufficient pressure built up, the fluid check valve deflected & the fluid under pressure flows in the pipeline. When the brake pedal release, the spring force in the master cylinder moves the piston towards right. This same force of spring keep the check valve pressed on it’s seat for sometime, there by delay the return of fluid into compression chamber. This delay causes vacuum in compression chamber & there may be chance of air leakage into the system. This vacuum is destroyed by entering the fluid from reservoir through intake port & holes in the piston which deflect rubber cup & enter in compression chamber.