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Module 2 - Hydraulic System Components (Page 2 of 5)

Hydraulic System Components

Master cylinder - The master cylinder is the central component of the brake system. It must perform these basic functions:

1. Act as a reservoir for the brake fluid

2. Contain the piston mechanisms to actuate the brakes.

3. Allow excess fluid to return to the reservoir

4. Maintain a residual system pressure in the brake system.

Master Cylinder Design and Configuration

The most common design for master cylinders is the dual system tandem master cylinder. This system divides the entire brake system into two seperate and independent sections. If one system should fail, the other system will continue to provide braking.

The two circuits in a dual system tandem master cylinder can be divided in two ways:

1. Front brakes / rear brakes - This is the most common system used in most cars and trucks. See diagram below. Quite simply, one of the two pistons inside the master cylinder supplies the front brakes. The other piston supplies the rear brakes.

2. Diagonal Split - This master cylinder arrangement which is pictured below is not as common as the front/rear split system pictured above. One of the two pistons inside the master cylinder supplies the left front and right rear brakes. The piston in the master cylinder supplies the right front and left rear brakes.

This system works well on vehicles with a low center of gravity. It also works well when a vehicle has very even weight distribution (sports cars).

Dual Tandem Master Cylinder

Tandem master cylinders have two separate pistons and two separate reservoirs.

There are two pistons inside a dual tandem master cylinder. One is called the primary piston, and the other is called the secondary pistons. Each piston has a set of sealing cups.

If one circuit fails, the other circuit will continue to provide hydraulic pressure. Study the diagram below. Diagram "A" shows a master cylinder with the brakes unapplied.

Diagram "B" shows the master cylinder with the brakes applied. There are no leaks. Notice that the primary piston (the rear one) does not mechanically apply the secondary piston. As you should be able to see in diagram "B", brake fluid gets trapped between the primary and secondary piston. We know that fluids cannot be compressed. So, when you step on the brakes, the trapped brake fluid applies the secondary piston.

In diagram "C", the secondary circuit (front one) is leaking. The brake fluid has leaked out of the secondary circuit reservoir which is nearly empty. But the primary reservoir is still full. When you press the brake pedal, brake fluid is still trapped between the primary and secondary piston. But because there is a leak in the secondary circuit, the secondary piston bottoms out as the fluid is leaked out. When the secondary piston is bottomed out, the primary piston is still able to create pressure. The brake circuits that are supplied by the primary piston are still fully operational.

In diagram "D", the primary circuit (rear one) is leaking. The brake fluid has leaked out of the primary circuit reservior which is nearly empty. But the secondary reservoir is still full. When you press the brake pedal, the brake fluid that is trapped between the primary and secondary piston leaks out. But the primary piston is pushed forward so that it mechanically actuates the secondary piston. In other words, the primary piston bottoms out against the secondary piston. Therefore the secondary piston is applied and the brake circuits that are supplied by the secondary circuit are still fully operational.

In the picture below, notice the secondary piston has a long rod protruding from the front of the piston. The purpose of this pushrod is to mechanically actuate the secondary piston in the even that the primary brake circuit develops a leak. See above diagram and explanations.

Module 2 (Page 2 of 5)