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In the old days, the hydraulic brakes in automobiles were simple systems. Here’s how they worked: when you pushed on the brake pedal, it pushed a piston that hydraulically moved the brake shoes on all four in wheels -simple as that. The only issue for some is that it sometimes it took a good amount of foot strength to stop larger vehicles. This was a problem and Detroit soon had its engineers starting to experiment with “power brakes,” meaning a mechanism to help the driver activate the brakes.  

After several years of production of early power brake designs, the industry settled on a design in the 1940’s that used engine vacuum to help the driver with braking. This system is still used today in most vehicles. It is referred to as “vacuum-assisted braking” or just “power brakes.”

How Vacuum-assisted brakes work
Basically, the brake booster in your car helps multiply the pressure you apply to the brake pedal. The result is that when slowing or stopping your car, you don’t have to push very hard on the brake pedal.

To explain the operation of a brake booster, we consulted Metro Kia of Atlanta, a local Kia dealer in Cartersville, GA., and this is what we learned. Brake boosters are a large, metal pancake-like devices mounted on the firewall right in front of the driver.

Inside the structure (see picture), a big flexible diaphragm divides the booster into front and rear chambers. On the outside, a thick hose connects the booster front chamber to the intake manifold as a source of vacuum. A push rod runs through the center of the booster. On one end, the rod connects to the brake pedal and to the brake master cylinder at the other.

The brake master cylinder attaches to the front of the brake booster. On a conventional booster, at the center of the push rod, you’ll find a normally open valve that allows vacuum to enter the rear chamber. Also, the rear of the push rod works as a normally closed valve to keep atmospheric pressure out of the rear chamber until you push down the brake pedal. Thus, both the front and back chambers have vacuum in them.

When you step on the brake pedal, you also push on the rear valve and center valves. So, the rear valve opens, allowing atmospheric pressure to enter the rear chamber. This pushes on the push rod which activates the brakes without much effort from your part.

Symptoms of a broken brake booster
There are three common signs that your brake booster may have failed. Let’s look at each one.

  1. You need more effort to apply the brake than usual. This could mean that the vacuum to the booster has been interrupted by a broken hose or some other failure.
  2. The brake pedal doesn’t return to its original position. When you take your foot off the brake pedal, it should return to its normal position. If it stays down by the floor, the vacuum booster may have failed.
  3. The engine rpm goes down when you depress the brake pedal. If you have a big vacuum leak inside the booster, it may affect your engine during idle. The test is simple. Let your car idle and then step on the brake. If it feels funny and the engine rpm changes significantly, the booster may be at fault.

Repairing a broken brake booster
Because of the critical task that a brake booster performs in your car, it is not a good idea to perform a DIY repair. This is a time that you should get a certified mechanic to replace the booster and bleed the brake system. Then you will know that it has been done correctly and the car is safe to drive.


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