How do you test a Power Brake Booster?


If you begin to notice that your brakes feel "spongy" then, there are likely issues with the power brake booster in such a car. See how to test it here!

When a vehicle is operating normally, most drivers and car owners never really border to be concerned about the inner working of their vehicle’s brake system. But whenever they press the brake pedal but notice that the vehicle isn’t slowing down, that’s when it always strikes everyone’s mind. But no more need for unnecessary panic as has got you covered

The power brake booster is one component that is usually first suspected as it used to be the leading culprit behind brake system, failure in old days’ SUVs, trucks and cars. It is normally used to brake fluid to the entire brake lines, which then permits the entire brake system to work smoothly and effectively.

Whenever this power brake booster is bad, it usually leads to some sort of “soft brake” problem or sometimes a total brake system failure. In this post, we have discussed some of the critical parts that work together within a vehicle’s brake system as well as how to diagnose and also determine whether the power brake booster is the source of a bigger problem or not with a car’s brake system.


Many drivers and car owners don’t usually care to know how the brake system in their car works until they face issues

Power Brake Booster – How does it really work?

It is important to first explain how the brakes in a car work before one could really understand how this Power Brake Booster component fits into our modern-day braking system.

In order to bring a vehicle to a halt, 3 scientific principles are actually needed to occur and they are;

  • Leverage
  • Hydraulic pressure
  • Friction

Each of these 3 operations must really work in tandem to truly bring a car to a standstill. Now, the brake booster comes into the picture by supplying the proper hydraulic pressure required by the brake callipers to really apply pressure efficiently to the brake rotor and then create necessary friction which is usually established by the direct application of the vehicle’s brake pads to the rotor itself.

The Power brake booster also functions by helping to provide the needed force for the correct amount of pressure in creating an efficient application of braking force. It usually works by drawing its power from a sort of vacuum which is normally created by the car engine during its operation/running.


The Power Brake Booster helps to supply the correct hydraulic pressure needed by the brake callipers of a car

This explains why power brakes in cars only work while the car engine is started or running and doesn’t when shutdown. The vacuum is the one responsible for powering an internal bladder that usually supplies the required force to the hydraulic brake lines of the system.

If this vacuum is actually damaged, leaking or the power brake booster’s internal components are rather damaged, it wouldn’t work properly at all.

Top 3 Ways to test for Power Brake Booster failure

Use the 3 methods below to test for Power Brake Booster failure and confirm it is truly the source of the brake problems or not;

Method 1:

Follow the steps below;

  • Turn off your engine and pump the car’s brake pedal many times to eliminate any sort of vacuum that might be remaining inside the power brake booster itself.
  • Press and hold down the car’s brake pedal really hard with your foot for one last time while you start the car engine. Make sure to not release your foot during this process from the brake pedal.
  • You should normally feel a slight sort of giving in the brake pedal as you turn the car engine over if truly the brake booster is in good condition and working appropriately. This is due to the fact that on turning the engine on, the vacuum in the engine now supplies pressure to the brake booster itself.

Method 2:

If you tried method 1 above by following the steps but the brake pedal doesn’t move then, it is a clear indicator that your car’s brake booster isn’t receiving enough vacuum pressure from the engine. And in such a scenario, attempt this method 2 by following the below steps;

  • Let the car engine stay on and run for a few minutes
  • Turn off the car engine, then press the brake pedal repeatedly in a slow manner. On pumping in the first time, it should feel very “low” – meaning that there is not much pressure resistance present. But as you continually pump the brake pedal, you should feel the pressure become firmer with each press, which will be an indicator that there is no leaking issue with the brake booster.

  Three Tests to Check Brake Booster (remake!) - VLV.4

Method 3:

If you tried Method 1 & 2 above and the two tests were passed then, there are just 2 more components that need checking.

  • Booster Check valve: You need to inspect the brake booster’s check valve. This valve is usually located on the brake booster itself directly. In order for you to easily locate it, you can consult your car repair/owner’s manual or search online. Remember though, to disconnect the vacuum hose because it will be connected to the car engine’s intake manifold component. If things are fine, air shouldn’t be flowing with pressure being applied. If you notice however that air keeps flowing in both directions or you find it difficult to blow air through then, the valve is likely damaged and your power brake booster will sure need a replacement.
  • Vacuum: For a power brake booster to work normally, a little amount of pressure is sure needed. You can test for the vacuum to verify if truly there is a minimum of 18 inches (at least) vacuum pressure present and no leaks.


If you truly read all these steps and recommendations but feel uncomfortable completing the different tests, your best option might be to just get a trusted mechanic to come to do a complete brake system inspection/checkup on your car.

We highly recommend that you avoid driving a vehicle that is given unclear signs, especially with its brake system. The brake system is not one part of a car that can be ignored or managed at all as it is the crucial part for bringing a vehicle to a halt when in motion.

Meanwhile, if you want more tips and advice like this which will help you keep your car in good shape for a long time then, navigate here: -> Car Care

Oluwaseun Solomon