Br5-Power Assist Diagnosis and Service

Br5-Power Assist Diagnosis and Service


NAME ______

Description: If you've ever opened the hood of your car, you've probably seen the brake booster. It's the round, black canister located at the back of the engine compartment on the driver's side of the car. Back in the day, when most cars had drum brakes, power brakes were not really necessary -- drum brakes naturally provide some of their own power assist. Since most cars today have disc brakes, at least on the front wheels, they need power brakes. Without this device, a lot of drivers would have very tired legs. The brake booster uses vacuum from the engine to multiply the force that your foot applies to the master cylinder. This is accomplished using either a vacuum boost or hydraulic boost system.

Vacuum Booster

Most light duty passenger cars and trucks use a vacuum booster. This can be located directly behind the master cylinder. The engine supply’s vacuum through a hose and just before it gets to the booster, it must go through a 1 way check valve as shown below


To check the operation of a vacuum booster, perform the following:

  1. Pump the brake pedal until all the reserve is gone (pedal should be hard)
  2. While holding steady foot pressure on the pedal, start the engine, you should feel he pedal drop about 1-2 inches, if so the booster is in working order.

Locate 2 shop vehicles that will start and run and check the boosters for proper operation

Instructor OK ______

Vacuum booster check valve

If the engine is turned off, or if a leak forms in a vacuum hose, the check valve makes sure that air does not enter the vacuum booster. This is important because the vacuum booster has to be able to provide enough boost for a driver to make several stops in the event that the engine stops running -- you certainly don't want to lose brake function if you run out of gas on the highway.

  1. Locate two vehicles with vacuum check valves, and remove the valve and show the instructor.

Instructor OK ______


Hydro boost does the exact same job as the vacuum boost except it uses hydraulic pressure from the power steering system to help apply the brakes. This system is used on medium duty trucks and some heavy duty applications.

If the hydro-boost system isn’t working, always check the hydraulic system fluid level, and belt tension before assuming the hydro boost unit is at fault.


Caution: some hydro boost units have an accumulator (blue can), these store hydraulic pressure in the event the engine stalls so the driver will still have some power assist, always pump the brake pedal with the engine OFF several times to deplete the pressure before working on the hydro boost system.

ASE test Questions

  1. Technician A says if the brake pedal drops when the engine is started while holding your foot on it, then the brake booster is faulty. Technician B says, the check valve is a 2 way valve. Who is correct?
  2. Tech a
  3. Tech B
  4. Both
  5. Neither
  6. Vacuum assist brake boosters operate by using
  7. Power steering fluid
  8. Brake fluid
  9. Engine vacuum
  10. Engine Oil Pressure
  11. The driver of a hydro-boost equipped vehicle notices the steering wheel is hard to turn, and the brake pedal is very hard. What is the most likely cause?
  12. Poor engine vacuum
  13. Low brake fluid
  14. Worn bushings
  15. Power steering pump belt is broken