Why Air Brakes Won’t Fail

From a mechanical standpoint, modern air brake systems are unlikely to fail.

Air Brakes Won’t Fail | Air Brake Smart


Updated Jan 2021

Closed Caption


Hi there smart drivers Rick with Smart Drive Test talking to you today about why air brakes won't fail.

In the last decade, we have equipped vehicles that carry our most precious cargo with air brakes.

And if air brakes failed they wouldn't be on those vehicles.

In the last two decades improvements in technology and the increased numbers of fail-safes on air brakes have more or less made these bulletproof.

The only failing of an airbrake system in this day and age is brake fade, and drivers have to work pretty hard to experience brake fade on an airbrake equipped vehicle.

Stick around to the end of the video, and we'll tell you which vehicles carry our most precious cargo.

PASS CDL Air Brakes First Time - take this comprehensive course!
A course for truck, bus & RV drivers operating a vehicle fitted with air brakes.
This self-paced course gives you:

• more than 100 practice test questions with feedback;
• EASY step-by-step instructions how to conduct a pre-trip inspection on an air brake system;
• DETAILED rules to use air brakes and brake downhill with a large vehicle safely;
• step-by-step instructions and precise wording for pre-trip inspection of trailers with airbrakes;
• explained SIMPLY to you is the tractor protection system and its components.

 Illustrations, checklists, & cheat sheets ensure your success 

$85.00 USD each


Hi there smart drivers, welcome back Rick with Smart Drive Test talking to you today about why air brakes won't fail.

As part of the requirement in both the United States and in Canada and other parts of the world, drivers have to take an airbrake course.

In those airbrake courses that were developed in the 1970s, many drivers are taught that air brakes are dangerous.

And that air brakes could potentially fail at any given time.

Unfortunately that is no longer true!

With advances in technology, modern air brake systems are more or less bulletproof.

And I would argue that the time has come for a revamp of all of these airbrake courses, because drivers do not need to know the technical inner workings of an airbrake system.

So what we're going to do today, we're going to go over why air brakes in this day and age are unlikely to fail.

Air Tanks (Air Reservoirs)

First failsafe of the airbrake system that will prevent it from failing are the air tanks themselves.

The air tanks on these systems are capable of holding enough air for 6 to 12 full brake applications.

So if the entire system shuts down, there's still enough volume of air in those air tanks to make 6 to 12 full brake applications and to stop a fully-loaded commercial vehicle equipped with air brakes.

You don't need a full brake application to bring the vehicle to a stop, so there's plenty of air in those tanks that if the compressor falls off the side of the motor or the whole system shuts down, there's still enough air to bring that vehicle to a stop.

One Way Check Valves

The one-way check valves at the entrance to the primary and secondary systems are the next failsafe on the system.

These prevent air from bleeding out of the system in the event there is a system failure or a break in the line between the compressor and the air tanks.

So the air can only move one way through the system.

It's unlikely, that if the compressor stops working or the main discharge line is broken, the air is going to bleed out of the system because of those one-way check valves which prevent the air from coming back through the system.

It only goes forward in the system.

Dual Air Brake System

The one-way check valves lead us into the next system and this system is found not only on air brake systems, but on your personal vehicle as well.

All braking systems are divided into two independent subsystems: if one system fails, the other will continue to work normally so long as the air compressor continues to pump air, which is rare that the air compressor will fail.

Even on a hydraulic system on your passenger vehicle, if you open the cover off of the master cylinder, you will see two chambers in there.

Since the late 1960s, all hydraulic braking systems have been divided into two independent subsystems - one for the front, and one for the rear.

An airbrake system is the same, it is divided it into two independent subsystems: the secondary, and the primary.

The secondary runs the steer axles and the primary runs everything behind the driver.

And it is the most prominent fail-safe system of an airbrake system, but there are more.

The air compressor on an air brake equipped vehicle provides ample air to operate the air brakes, but also the many accessories on the vehicle.Air Compressor

The air compressor is another failsafe on the system.

These air compressors pump more air than could possibly be required and we see that on modern air brake systems because the air compressor pumps enough air to run all of the air accessories on the vehicle, whether it's a bus or a truck.

And one of the most demanding systems that uses air pressure is the air-ride suspension, because unlike a passenger vehicle that has airbags and it is just a balloon that you pump up and it just pushes down like as if you were going to squish a balloon.

On a tractor trailer or a bus there is air constantly being evacuated out of the airbag system when it goes over a bump, it actually releases some of the air and then it fills back up.

The compressor is constantly pumping air to fill that air ride suspension.

As well as other accessories on the vehicle: air ride seats, or in the rare case of windshield wipers, gear selectors and those types of things.

The air pressure is used for a lot of other accessories, so the air compressor is another failsafe on the airbrake system because it can supply enough air to not only run the air brakes, but all of the accessories on the vehicle.

Pressure Protection Valve

The next failsafe--saying that about air accessories on the vehicle--there is a pressure protection valve on the system.

So what happens is, if the system pressure starts to drop between 60 and 90 pounds (psi - pounds per square inch) the pressure protection valve will shut off air to the accessories and direct air just into the air brakes.

So that's another failsafe, is the pressure protection valve which redirects air from the accessories into the air brakes if there's a loss of air pressure between 60 and 90 pounds.

And it varies a little bit depending on what vehicle you're operating.

Low Air Warning

One of the most prominent fail-safes on a modern airbrake system is the low air warning device.

And in this day and age, they're all a light and a buzzer - so they're audible and they're visual - you can see it and hear it.

And the low air warning is pretty annoying - it's that, you know, most annoying sound in the world.


That sound -we all know that sound! Man that's annoying!

The low air warning comes on at 60 pounds and the reason it comes on at 60 pounds is because at 60 pounds there isn't enough air pressure in the system to hold the gigantic springs off in the caged position which activate the parking brakes and also work as the emergency brake.

So at 60 pounds the low air warning comes on.


And in keeping with the low air warning, all of the air pressure on an airbrake equipped vehicle is plumbed through the dash and even over the sound of all of the road noise, and the diesel engine, and people talking--if you're driving a bus, driving a truck--you can hear air leaks on that system.

So even a minor air leak is prominent and you will be able to hear it.

So that's another failsafe, is that the driver is actually paying attention to the vehicle and listening to what's going on and hearing air leaks in the system.

The yellow parking control valve on the dash applies and releasing the spring brakes on the truck or bus.Spring Brakes

The most prominent failsafe on the airbrake system, next to the dual air brake system-- the division of the system into two independent subsystems--are the spring brakes.

Most of the time the spring brakes are used for parking, but in the event of an emergency and there's a loss of air the spring brakes are going to activate because while the vehicle is going up and down the road, those giant springs are held in the released position by air pressure.

And unlike the movie Terminator, which can see here in the clip when he rips the air tank off the bottom of the bus and they lose the brakes, that will never happen on a real airbrake system.

What happens is that you lose all the air pressure in the system, the springs expand, and engage the brakes.

So most of the time those spring brakes are used for parking, but in the event of a catastrophic air loss in the system, the spring brakes will engage and the vehicle will come to a screeching halt.

And oftentimes that happens between 20 and 45 pounds per square inch.

So if you're the driver and that low air warning comes on and you're not paying attention--and usually what I say to drivers is that you have to be sleeping, stoned, and really drunk not to hear that air loss or hear the low air warning and not know that it's going down between 20 and 45 psi.

Because if you don't pay attention, you are going to be doing a bug impression on the inside of the windshield because that vehicle is going to come to a screeching halt!

In-Cab CDL Air Brake Checklist This checklist will ensure that you know the numbers and pass your CDL Air Brake In-Cab Pretrip Inspection first time!

* FEAR :: Correct procedures for the MUST-DO IN-CAB CDL AIR BRAKE INSPECTION;

* SELF-CONFIDENCE :: Detailed sample test shows you how the tests are completed step-by-step;

* EMPOWERMENT :: Easy to use guide cuts through all the rigamarole of CDL manuals.

Tractor Protection System

The next failsafe on a truck designed to pull trailers with air brakes is the tractor protection system.

The tractor protection system as it's called--this is one of those that the engineers actually got right.

The tractor protection system protects the tractors air supply in the event that the trailer falls off the back of the truck.

Just quickly the tractor protection system consists of two valves:

1) the trailer air supply valve on the dash and

2) the tractor protection valve which is called the same thing as the track protection system just to confuse drivers when we're teaching it to them.

Anyway, the tractor protection valve in the back is kinda like the guard, it monitors air pressure into the trailer.

If it detects catastrophic air loss in the trailer, it'll tell the trailer air supply valve on the dash to shut off air to the trailer to protect the air supply in the truck.

It's another failsafe on 18 wheelers and trucks designed to pull trailers equipped with air brakes.

Automatic Slack Adjusters

[DRUMROLL] Drumroll...the last failing of an airbrake system was manual slack adjusters.

Manual slack adjusters had to be adjusted by the driver to keep air brake systems in adjustment and working properly.

And up until the mid-1990s and early 2000s, 75-80% of trucks during the annual blitz on the part of the CVSE (Commercial Vehicle Safety & Enforcement) the MTO (Ministry of Transportation of Ontario) and the DOT (Department of Transport) in the United States found that trucks were put out of service because brakes were out of adjustment.In the last 2 decades, air brakes have been fitted on school buses because these braking systems are reliable.

In the early 2000s, the province of Ontario, Canada did a study to look at vehicles going over the scales and looked at how many of those vehicles were equipped with automatic slack adjuster - so six years after it became law that automatic slack adjusters had to be fitted on new trucks and trailers and buses, they found that 96% of vehicles had automatic slack adjusters.

So here almost two decades plus years later most vehicles have automatic slack adjusters.

And actually, if they go into a shop for their annual inspection and are found to have manual slack adjusters they have to be swapped out, for that vehicle to pass its annual inspection.

Automatic slack adjusters are on most vehicles in this day and age...actually all vehicles in this day and age - they won't pass their annual inspection.

Automatic slack adjusters are reliable and work well to keep the brakes in adjustment.

And as I said, manual slack adjusters were the last failing of the airbrake system.

And essentially automatic slack adjusters are just a ratchet mechanism that when the brakes are applied it clicks over to the next ratchet position.

It's kind of like a boat ratchet....click, click, click...and when the brakes wear and there's a certain amount of slack and the pushrod comes out of the brake chamber a certain distance, it will ratchet over to the next position and keep the brake adjustment.

And these are effective - quite effective actually.

They've made air brakes almost more or less bulletproof.

It was the last hole in the air brake system that has now been corrected.


And finally, the "proof in the pudding" of why air brakes are more or less bulletproof in this day and age.

In the last five to ten years, school buses, that carry our most precious cargo, have all been fitted with air brakes.

And air brakes are more or less bulletproof because they're on school buses.

Question for my smart drivers:

Have you taken an airbrake course in the last decade and were you taught to believe that air brakes would fail and we're not reliable?

Leave a comment down in the comment section, all helps us out.

Log in to comment