More and more parts of our modern cars are getting some sort of assistance to make driving easier. There is power steering, lane keep assist, power windows, and boosted braking systems.
The vacuum brake booster is a loyal system that works behind the scenes to translate a gentle tap onto the brake pedal into a more usable force. This saves you the pain of having to push really hard on the brake pedal to bring a car to a stop.
An excellent way to test how much force you need to brake on your own is by trying the brake pedal with the car off. You will feel as if the pedal is solid and unmovable.
However, it can still move. It is designed in a fail-safe manner that lets you stop the car even if you lose your brake boost.
How Do Brake Boosters Work?
A brake booster is a power multiplier system that sits between the brake pedal and your master cylinder. It does this by getting some form of powered assistance from the car’s engine.
In petrol-powered cars (at least naturally aspirated ones), the brake booster gets this power from a pressure differential it creates using the engine’s vacuum.
Every time you tap on the brakes, a valve connecting the master cylinder to this vacuum source opens. This allows the vacuum differential to move the diaphragm in the pressure boost cylinder to one side. This will, in turn, push a lever that pushes the master cylinder piston.
The extent to which the valve opens is proportional to how hard you press onto the brake pedal.
Why Do Diesel Engines Need a Separate Vacuum Pump?
Remember we specified petrol engines get their vacuum from the motor? Well, this is because diesel-powered engines run a separate vacuum pump to generate their vacuum effect.
Even though diesel engines can create a vacuum (after all, they still suck in air), it isn’t as strong as what a petrol engine generates. This is because they don’t have a carburetor or a throttle body to create ‘chocked flow.’
By design, diesel engines run on a fully open-air intake. They regulate their RPM by altering the amount of fuel injected into the cylinders. The lack of a device to restrict how fast air from the atmosphere feeds the cylinders in the intake stroke drastically reduces the force of the vacuum.
Turbocharged and Supercharged Vehicles Also Need a Vaccum Pump
Turbocharged or supercharged engines, be it petrol or diesel, also need a vacuum pump. Tapping into the intake manifold for pressure would sabotage the supercharger or turbocharger.
Such engines tend to have a positive pressure at the manifold since the super or turbochargers are actively cramming air. Any opening will negate this benefit by letting all that boost leak away.
Electric or Hybrid Vehicle Need an Extra System
Finally, if you drive an EV or a hybrid, your car doesn’t use a traditional brake booster. This is because electric vehicles don’t have an engine to generate the vacuum a brake booster needs.
Hybrids that run on pure electric power at low speeds also need a different brake boost system since the engine won’t be running at such speeds.
What Are the Symptoms of a Bad Brake Booster?
Now that we’ve learned how brake boosters work, it is time to look at how a failing brake booster affects your car and how it drives. Almost all these signs hint at reduced brake efficiency. They include:
A Brake Pedal That is Harder to Press
Since the brake booster’s primary role is to amplify the force you apply to the brake pedal, a failing or failed booster will leave you on your own.
As such, you will need more energy to depress the brake pedal to the correct depth. You will notice this very fast as it will limit your ability to stop your vehicle.
A Longer Braking Distance
No matter how strong you are, chances are you won’t apply enough force to stop a car in time, especially in an emergency.
The brake’s clamping force will reduce, and your car will need a longer distance to stop. Moreover, additional features like automatic braking or any assisted braking and brake force distribution systems won’t work.
This makes your car harder to stop and less safe than the manufacturer designed it to be.
ProTip: Worn-out or bad-quality brake pads can also affect your stopping distance. Check yours frequently and ensure you use OEM or better than OEM replacements.
The Brake Pedal Rides Higher Than Before
Sometimes, if there are problems with your booster, the pressure from the vacuum sources pushes the pedal up.
Not only will this pressure resist you, but it will also make the brake pedal ride higher than usual.
Spongy Brakes
Know that spongy feel that creeps into your brake pedal when you haven’t bled your brakes well? The same could happen if your brake booster fails. This is common if the check valves in the system are the problem.
A faulty check valve could let air bubbles into the braking system. Since air is compressible (unlike brake fluid), you will lose some braking force at every step on the pedal.
This not only makes the pedal spongy but also reduces your braking efficiency leading to longer stopping distances. Your brakes could also be spongy if you opened the hydraulic system but did not bleed the brakes.
A Rough Idling Engine or Frequent Stalls
If your brake booster generates power from the engine’s vacuum, you will get tell-tale vacuum leak symptoms – assuming the booster is leaking.
We did a post on what a vacuum leak is and how to detect it. Your brake booster could lead to all those symptoms, especially if it’s leaking – not just jammed.
The most significant symptoms you will get is a car that idles very rough (vibrations you can feel in the cabin), behaves like stalling (or even stalls) when you put it in drive (especially with the engine loaded, like the AC on.)
A Hissing Sound From Under the Hood
Another good sign to solidify all the above symptoms is a hissing sound from under the hood. Since brake boosters create pressure differentials, you hear a hiss as air rushes in or out of the system.
This is a tremendous reinforcing symptom, primarily if your car uses a vacuum pump to power your brake booster instead of tapping into the engine manifold for the vacuum.
A Brake Warning Light or an ABS Light
Some cars have sensors monitoring how your brakes perform. If this is the case, you will get a brake warning light on the dash (a handbrake light that doesn’t go away when you disengage the emergency brake)
If your car has ABS or any other braking assistance system, it has sensors that monitor the impact of a braking force on your tires. If it sees inconsistencies between what it expects and what happens, it surmises that there’s an error somewhere. So the ABS or other related error lights will light up on the dash.
How to Test Your Brake Booster
Here is an excellent way to test your brake booster system regardless of how the car generates a boost.
- With the car in the off position, pump the brakes a couple of times until they get stiff. This will deplete any form of assist vacuum the system had in store.
- Depress the pedal firmly and turn on the car
- If the system is effective, you should feel the brake pedal drop off under your pressure before slightly pushing back against your foot.
- If the pedal remains at the same position under your previous pressure, then the boost system isn’t working right.
- Also, there is a problem in the braking system if the pedal falls straight to the bottom and stays there.
Different Types of Brake Boost Systems and How They Work
As we’ve already seen, different types of cars need different types of brake boost technologies. As such, there are multiple types of systems to get the job done. Currently, the market has three main systems.
Vacuum Boosters
These are the most common systems in the market right now. Traditional petrol engines use them because they already have a ready vacuum source in the engine.
Diesel, supercharged and turbocharged engines use the same systems only that they get their vacuum from an electric or mechanically driven vacuum pump. Some hybrid vehicles could also use this approach.
Hydro-Boost Systems
This approach gets the pressure to push the master cylinder from the hydraulic pressure generated by the power steering pump. It is a great way to reduce components, especially in vehicles with hydraulic (not electric) power steering assist.
Electro-Hydraulic Boost
This form of brake boost is becoming more common as it is easier to use electronics and sensors to control it. The car uses an electric motor to generate high-pressure gas that pushes against the master cylinder on demand.
Whether internal combustion, hybrid, or electric, most modern vehicles prefer this option, especially if they have integrated antilock brakes, emergency stop, and brakeforce distribution systems.
It is also common in brake-by-wire vehicles that use sensors on the brake pedal to determine its position and relay that information to the fully electric brake system that then applies the correct braking force to the calipers.
Why Do Brake Boosters Fail?
Brake boosters are resilient. They’re designed even to outlast a car. Moreover, since they’re often stashed away at one of the safest spots under the hood, they are hard to damage. Nonetheless, there are still a couple of things that can bring them down.
ProTip: While some parts of any power brake system are virtually damage-proof, they could still fail if a car sits for very long or is involved in a significant accident. In this case, you will have to either write off the car or do a complete overhaul of the braking system.
Failure Points in Vacuum Brake Boost Systems
Here are the common failure points in a traditional vacuum brake boost system.
A Leaking Pressure Accumulator
The pressure accumulator holds enough pressure to give you assisted braking for at least four or so complete stops.
This is why you can still brake a couple of times even with your engine off.
A leak in this accumulator prevents it from building up enough pressure. It could also mean the accumulator leaks over time, leaving you with no brake boost, especially when you step into the car after parking it for a while.
A Failing Diaphragm
Your booster uses a diaphragm to translate all that vacuum pressure into a real boost. If the diaphragm is cracked or leaking, some of the force will leak through.
You could have some air leaking into your master cylinder and the brake fluid giving you spongy brakes in worse cases.
However, diaphragms rarely fail since they are inside a closed system.
A Damaged Check Valve
The check valve maintains pressure in the booster. It works by letting the vacuum in and ensuring it doesn’t go out (one-way valve.)
This ensures that you will have a vacuum in your accumulator even with the engine off or when you have a small leak in the delivery plumbing. This is crucial to letting you brake a couple of times, even without a new vacuum supply to the system.
The check valve also releases extra pressure after you let go of the brake pedal. It does this by drawing out excess air via the vacuum hose. If it doesn’t, your system will be overpressurized and will fail faster.
It also keeps air from seeping into the brake lines via your master cylinder. If it fails, you could:
- Have gradual and catastrophic failure to the entire system due to pressure overload
- No consistent boost since the accumulator can’t hold vacuum
- Spongy brakes when the system lets air bubbles into the brake lines
- Your brakes are erratic. Sometimes soft and boosted, occasionally hard and unboosted.
Loose, Cracked, or Blocked Vacuum Hose
Since your brake boost works on a vacuum, anything that blocks its access to its ‘life-giving juice’ could have negative impacts.
For instance, a loose connection that leaks vacuum before it gets past the check valve will reduce how much boost your brakes can provide.
Similarly, cracks or blockages within the delivery vacuum hose will also have noticeable negative impacts.
Low Intake Vacuum on The Engine
This, for once, is a problem not related to your braking system. Your boost system could be exemplary but not performing because it’s not getting enough vacuum from the engine.
Problems within the engine or its intake system could lower the vacuum. Luckily, your engine will run rough, and you will have other concerns over there that need urgent attention.
You can check our post on vacuum leaks at the engine to learn more about this.
A Faulty Vacuum Pump
Finally, if your car gets its brake boost vacuum from a pump, you will have problems if the pump is faulty.
Sometimes, the pump could be OK, but its drive system isn’t working. Mechanically driven pumps get power from the engine via a belt and pulley system. A worn-out belt that’s slipping could lower performance, while a missing belt that broke off will give you nothing at all.
On the other hand, electric pumps could either seize up or have a blown fuse/relay causing the problem.
Failure Points in Hydro-Boost Systems
Hydro-boost systems also fail. However, their failure points are different. These include:
A Leak in the Pressure Accumulator
Just like the vacuum-powered system, your hydro-boost system also stores some pressurized power steering fluid in an accumulator. This is to give you at least four power brake chances even with the engine off.
In case the accumulator is leaking or damaged, you won’t have this freedom. Moreover, the system might not produce the necessary pressure for full brake assist even when driving.
Worn out Spool Valve or Piston
Your spool valve is in charge of passing on pressure to the master cylinder. If it is worn out, it might leak pressure or create a lot of friction in the system.
This not only bleeds pressure but also makes the brakes return slower than usual. As a result, you end up with grabby brakes.
A Slipping Power Steering Pump Belt
Since your power steering pump could be powered by a belt connected to the engine, chances are a slip in the belt will lead to lower pressure in the system.
Luckily, you can pinpoint this fast since your power steering won’t have sufficient assistance too.
Low Power Steering Fluid
A leak in the hydro brake boost system or the power steering system will lower the power steering fluid.
Very low fluid levels introduce more air into the system, making it harder to build up enough pressure to sufficiently power your boosted brakes and power steering.
You can check the power steering fluid tank levels to confirm that it is at the proper levels.
If it is dropped, check your plumbing to the hydro-boost system and the power steering system to pinpoint and fix the leak before topping up your fluid.
Restricted Return Valve or Dump Valve
Hydro-boost systems bleed pressure through a return pipe and a dump valve. However, if these parts are faulty or blocked, they won’t bleed pressure back into the power steering reservoir once you release the brake pedal.
Consequently, your brakes could apply themselves even if you don’t step on the brake pedal. Moreover, they could also react firmer and produce a braking force that isn’t proportional to how far you depress the brake pedal.
Problems in Electro-hydraulic Braking System
This is the latest and most sophisticated brake boost system in the market yet. Though more resilient, it means it has a couple of possible failure points. These include:
A Leaking Pressure Accumulator
Electric braking systems also have an accumulator. It works the same way accumulators in the above two systems work.
The problems will be similar if you have a leak.
A notable difference is these systems run the accumulator at extremely high pressures. So instead of priming it just for a few stops, it keeps it topped up and runs the pump as soon as pressure levels drop below a given level.
A Faulty Pressure Switch Sensor
Your system uses a pressure switch sensor to control the pump in this system. When pressure drops below the set level, it kicks on and replenishes the reservoir.
If this sensor fails, it will either:
- Not turn on the motor on time leading to a loss in boost pressure
- Keep the pump running for too long. This will overpressurize the system beyond spec damaging some components.
A Bad Pump Motor Relay or Fuse
Sometimes, everything could be working fine, but you are not getting boost just because your components aren’t powered.
If your system runs on an electric system, check your fusebox and confirm that the power brake fuse is intact. You should also check the relay to ensure that it is not damaged.
A Failed Pump Motor
While the motor is designed to last the car’s lifetime, it could be damaged in an accident or case of an unexpected occurrence.
If the motor seizes up, you won’t get any pressure for the boost at all. Moreover, it could keep blowing fuses.
You can quickly confirm the motor is the problem by checking that it’s getting 12V power at the terminals. If it doesn’t, the problem could be in the wiring or at the contacts.
ProTip: Remember to have someone pump the brakes a couple of times to deplete reserved pressure so that the vacuum pump motor tries to kick in.
A Bad Brake Position Sensor
Finally, if you have a brake by wire system, you will have a brake position sensor by your brake pedal.
If this sensor is faulty, it will relay flawed data to the ECU. As a result, the car will either brake poorly or erratically.
Is Brake By Wire Safe?
Yes, brake by wire is safe. However, with all the electronics in the way, you might be tempted to believe that you’ll have no brakes when things fail.
Just like power steering, the car still has a mechanical link to the braking system. If the system detects some erratic readings in one of its sensors, it will shut off. A fail-safe valve then bypasses all the electronics giving a direct link to the master cylinder keeping the brakes working as long as you haven’t leaked all the brake fluid.
This lets you use muscle power to firmly step on the pedal and bring the car to a stop without brake boost.
Can You Drive Without a Brake Booster?
Disclaimer: Just because you can doesn’t mean you should. We have brake boost for a reason. A car without brake boost is a hazard on the road. It could be technically unroadworthy.
You could drive without a brake booster. However, you should be aware that your car won’t stop as well. So be careful and drive at reduced speeds. Be ready to apply a lot of force onto the brake pedal to bring the car to a stop.
Make your calculations in advance. Even at low speeds, emergency braking won’t be there to get you out of a mess.
Note that the heavier the car, the harder it is to stop without brake boost. Don’t try this on heavy trucks, SUVs, or saloons, even if they are not loaded. You could get away without boost for a limp to the garage, but that is about it.
Will Power Brakes Work if the Engine Fails?
A power brake system with a perfectly working accumulator will give you at least four brake applications when the engine fails. Use these wisely. Apply the brakes gradually instead of braking and releasing since you lose some pressure every time you get your foot off the pedal.
In cases where an electric motor generates your brake boost vacuum or pressure, you might have some assistance for longer since your pump can still work off the car’s battery. It will drain the battery fast, though. This will still fail if an electrical problem made your engine fail.
Does Hydroboost Affect Power Steering?
If the system is working perfectly, there won’t be any effect on your power steering system.
However, a failure in one of the hydro-boost systems that waste the pressure in the power steering system will rid you of some (or all) power steering. The same will happen to your brake boost if there is a similar fault in the power steering section of the system. The only difference is you will notice it after a while since your hydro-boost stores some pressure in its accumulator.
Bottom Line
Brake boost is crucial to modern-day driving. Cars are heavier and have more safety features that rely on brake boost to keep you safe on the road. If it fails, your vehicle will not only be harder to control, but things like traction control that works by strategic brake force distribution won’t work.
Have an expert look into your car as soon as you suspect you have a problem with your brake boost system. This could save your life and money by keeping you away from unnecessary crashes.