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Brake Assembly Under The Hood
Brake Assembly Under The Hood

This is a describtion of what happens under the hood of your car when you press the pedal. It is a describtion of how pushing the pedal sends hyrdrolic fluid to the brakes. The description of what happens with the fluid at the pedals is described in later sections on this page.

The Master Cylinder

In cars, located in the engine compartment in front of your pedal is something called a Master Cylinder. The master cylinder's job is to deliver hydrolic fluid to the brakes and as a saftey procaution it must provide braking power even when there is a leak in one of the lines. Most cars have two different circuits of tubes for the hyrdrolic fluid. Then one circuit works only 2 tires. The front are on one circuit and the back on the other. The master cylinder is what will distribute the fluid through both.
Master Cylinder

The brake pedal is connected to a pivot point a few feet from above the pedal. It is located above its connection to the master cylinder. When you push the pedal is moves the bar on the pivot forward and that pushes another bar into the master cylinder. The rod that is pushed into the master cylinder pushes a piston in the master cylinder. There are two pistons with two springs, one in front of each. The first piston pushes fluid forward. Some of the fluid goes down into one of the brake lines. The other fluid pushes the back of the next piston. This second piston then pushes fluid down the second brake line. There is a brake fluid reservoir above the pistons. The reservoir is connected to the cylinder between each piston, so there are two places for the fluid to enter. This is important because if there is a leak the fluid will be able to go to a working piston, instead of it all going out one hole to both.



When you release the brake the springs push the pistons back into place. The reverse motion of the pistons creates a suction. This suction sucks the fluid back, in turn releasing the brakes.

Now in the case of the leak in the first circuit, the piston would push out the fluid and then push the second piston. The second piston would press fluid to the brakes and create braking power. If the second circuit had a leak, the first piston would push its fluid like normally into the first circuit.


Master cylinder with leak


The Combination Valve

Combination Valve


On cars with disc brakes in the front and drum brakes in the back a special valve is needed. This valve is located just after the master cylinder. It recieves the brake fluid after it is pushed the the master cylinder. The combination valve does a few things all in one assembly. It:
The Metering Section

In a car with disc and drum brakes this section is needed. Drum brakes once applied will return to there original position. In disc brakes once applied the brakes will release there grip on the rotor but they will remain right next to the rotor. If you applied your brake the disc brakes would end up locking before the drum brakes in the back. This valve makes sure will make sure the rear circuit is under a certain pressure before the brake force goes to the all brakes. So when you press the pedal the fluid first goes to the drum brakes so they are closer to the rotor. Then the force will go to all brakes and they will lock together.

The Pressure Differential Switch

This is a switch that will let you know on the control panel if there is a leak in the brake lines. It is simply a cylinder with a piston in the middle. When the brakes are normally activated the piston will remain in the center because of equal pressure in the brake lines. Once there is a leak the pressure will become unequal. When the pressure becomes unequal the piston will move to one side and activate a switch letting you know of the leak.

The Proportioning Valve

As in all cars when braking the front tires take more weight then the back doing. You can notice it when your driving and as you stop you feel your weight shift forward. Because more weight is shift to the front you need mopre braking power there. In a lot of cars they even design the front brakes to be larger then the back. Going back to the valve... it reduces the pressure of the fluid going to the back brakes. Now if this was not used the back brakes would lock up the tires before the front. This would cause a skid.

Power Brakes

Under Contruction

Now for information about the brake assembly at the wheel. Click Here.


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Pictures: Thanks to HowStuffWorks.com


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