• Master cylinder is a control device that converts non-hydraulic pressure (commonly from a driver's foot) into hydraulic pressure.
• 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 tandem master cylinder has two separate hydraulic chambers. This creates in effect two separate hydraulic braking circuits. If one of these circuits becomes inoperative, the other circuit can still function to stop the vehicle. Stopping distance is increased significantly, however, when operating on only one braking circuit. This is one of the vehicles' most important safety features.
• A simplified diagram of the tandem master cylinder is shown
• Under ordinary conditions the brake fluid will transmit pressure both to front as well as rear brakes, when the brake pedal is applied.

• However, when, say, the front brake lines are damaged, piston 2 will move till it comes up against stop (3). After this pressure will start building up in the space between pistons (1) and (2) and rear brakes are applied.
• Similarly when the rear brake lines are damaged, no pressure will build up in space between pistons (1) and (2). So piston (1) will move freely till comes up against (2). Further push at the brake pedal will move (1) and (2) together there applying front brakes.

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:5.1). 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:
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.

Tandem Master cylinder: Tandem Master cylinder ensures reliability with not much extra cost. In this separate lines go to different sections of the brake system, say, the rear & the front brake lines & it is so arranged that if the front brake lines are damaged, the rear brake will be still effective. Similarly, if rear brake lines is defective, at least front brake will be applied. A simplified diagram of the tandem master cylinder is shown in fig 7.1.