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The Most Common Failures Of A Brake Booster: Symptoms And Problems!

The brake booster is the heart of the brake system. When you step on the brake pedal, the brake booster multiplies the force of your foot, thus reducing the effort required to stop the vehicle. If this part fails, it could leave you injured in the middle of the road, so it is necessary that you know the common failures of a brake booster .

Today, we take it for granted that power brakes are a reality. This technology wasn’t always common: Until electric brakes became the norm in the late 1960s, many drivers relied solely on their leg muscles to provide braking assistance.

What is a servo brake or brake assist?

As mentioned, the brake booster reduces the effort the driver needs to apply the brakes. Brake power does this by applying force to the master cylinder, a device that distributes pressurized fluid to activate the brakes.

Brake Booster Types

There are three main types of brake boosters:

1- Vacuum actuated brake booster

Most brake boosters are vacuum. With this design, an internal diaphragm separates the two sides of the power brake . The two sides are sometimes called the vacuum chamber and the working chamber.

When the brakes are released, there is an equal amount of vacuum in both chambers. Depressing the brake pedal actuates a control valve that allows atmospheric pressure to enter the work chamber. As a result, a pressure difference is created that causes the brake booster pushrod to apply force to the master cylinder, creating brake assist.

2- Hydro-boost brake enhancer

Some vehicles use a hydro-boost power brake. With this type of brake booster, the power steering pump builds hydraulic pressure to provide braking assistance. The pump pressure acts on a power piston inside the hydro-boost assembly. The piston then pushes an output rod which applies force to the master cylinder.

3- Electronic brake booster kit

Electronic brake power packages are becoming more common. You’ll often find this design on hybrid and electric vehicles that have zero (or limited) engine vacuum and electric power steering.

There are a variety of electronic power brake designs in use today. Each of them uses a set of sensors (ie pedal travel sensor, speed sensors, etc.) to determine the operating conditions of the vehicle. An electronic control unit (ECU) uses that information to activate the electronic brake booster.

The brake booster and master cylinder then create the necessary hydraulic pressure to provide braking assistance.

Often the electronic brake booster is built into an assembly with the master cylinder and ECU. Other components may also be part of the assembly.

Signs and common faults of a brake booster or brake assist

If your car has a failed brake booster, you’ll probably notice one or more of the following symptoms that are signs of frequent brake booster failure.

That being said, when your brake booster fails, you’ll likely notice it right away, as this issue often produces one or more noticeable symptoms. Obviously, since brake power affects your car’s stopping ability, you’ll want to fix any issues immediately.

Well, the signs or common failures of a bad brake booster are the following:

1- Hard brake pedal

A hard brake pedal is the most common sign of a bad brake booster or power brake. When the brake booster fails, you no longer have brake assist, which means you’ll have to put in more effort to apply the brakes.

2- Increased braking distance

Without the brake assist of the Brake Booster, you will have to work harder to apply the brakes. This usually translates into an increase in stopping distance.

3- Hissing noise

A leaking vacuum brake booster in the housing or diaphragm can create a hissing noise. The noise may only be heard when applying the brakes .

4- Liquid leaks

Hydro-boost brake boosters use power steering fluid to provide braking assistance. As such, a leaking hydro-boost unit can develop visible fluid leaks. Electronic brake booster assemblies that are housed with the master cylinder can also develop leaks. This is a fairly common failure of a brake booster.

5- Bright warning lights

A problem with the electronic brake booster assembly may affect the operation of the anti-lock braking system (ABS). As a result, an assembly failure has the potential to trigger warnings such as ABS, traction control, and stability control lights.

It is also possible that a leak in the vacuum-actuated assisted brake is causing the engine to run underpower , which can trigger your check engine light.

How to Test a Brake Booster (Vacuum Actuated)

The steps to test a brake assist vary by vehicle and by the design of the brake booster. The information below is generic and is for educational and entertainment purposes only. Be sure to follow the factory repair information for the specific application.

With this in mind, we are going to discuss general brake booster test methods for a traditional, vacuum-operated unit.

1- Check the operation of the booster and the vacuum reserve

Vacuum brake boosters have a reserve to provide braking assistance if engine vacuum is lost. You can check that the brake booster is working properly and has an adequate vacuum reserve (indicating that it is not leaking).

2- Functional test of the pressure booster

  • Apply the brakes several times with the engine off to exhaust the vacuum reserve.
  • Keep the brake pedal applied and start the engine.
  • Once the engine starts, the brake pedal should come down if the brake booster is working properly.
  • If the pedal does not go down, check that there is an adequate vacuum supply at the brake booster. An adequate supply of vacuum indicates that the brake assist is probably faulty and must be replaced.
  • Since the brake booster is dependent on engine vacuum, inadequate vacuum supply can simulate a faulty brake power supply.

3- Check the booster vacuum reserve

  • Start the engine and rev it momentarily.
  • Let the engine idle for a minute, then turn it off for a few minutes.
  • Step on the brake pedal several times. If the booster holds vacuum, the first few applications should have power assist. Next, the pedal must be firm.
  • If there is no power assist, either the booster or your check valve is leaking.
  • You can check the valve by blowing through your inlet end. If you hear air go through the valve as you blow, the valve is bad and needs to be replaced.

Otherwise, the brake booster is probably faulty.

4- Replacing the servobrake (actuated by vacuum)

The tools needed to replace a power brake will vary, depending on the type of car you have. However, in general, you will need:

  • Pliers
  • Ratchet set (ratchet, sockets and extensions)
  • Repair manual or access to a repair database
  • safety glasses
  • screwdrivers
  • torque wrench
  • wrench set

5- Instructions for replacing the vacuum brake servo

Before you begin, keep in mind that all vehicles are different. The information below is generic and is for educational and entertainment purposes only. Be sure to follow the factory repair information for a specific application.

Repair manuals are helpful, but a subscription to a repair database is even better.

How to remove a vacuum brake booster?

  • Put on your safety glasses.
  • Use a wrench or ratchet to unbolt the master cylinder from the brake booster.

(In most cases, it is not necessary to disconnect the brake lines from the master cylinder. Brake lines are usually long and flexible enough to allow the master cylinder to be moved to the side, away from the brake booster. If you need to remove the lines, be sure to properly bleed the brake system afterwards, or you’ll have little to no stopping power)

  • Disconnect the pushrod from the brake pedal.
  • Disconnect the vacuum hose going to the brake booster.
  • Use a wrench or ratchet to remove the bolts that hold the brake booster to the bulkhead.
  • Remove the brake booster from the vehicle.

How to install a vacuum brake booster?

  • Compare the new brake booster to the old one to make sure they both have the same design.
  • Mount the brake booster in the vehicle.
  • Consult a repair manual or repair database to determine the proper torque for the brake booster mounting bolts. Then use a torque wrench to tighten the bolts to the manufacturer’s specifications.
  • Reinstall the vacuum hose that goes to the brake booster.
  • Reconnect the brake booster pushrod to the brake pedal.
  • Mount the master cylinder to the brake booster. Use a torque wrench to tighten the master cylinder mounting bolts to the manufacturer’s specifications.
  • Check the brake pedal height and free play after replacing the master cylinder. If necessary, consult a repair manual or repair database for the proper adjustment procedure.

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