Air Brake Relay – How it Works. Air braking systems and Commercial vehicles

Air Brake Relay – How it Works. Air braking systems and Commercial vehicles

Hello and welcome back to Educational Mechanics, I’m Matt and in this video were going to
look at a bit deeper into the air braking system in particular at the air brake relay. So hold on to your seats! A quick recap on what we already know. But if you want a bit of a refresher, click
here to watch our “Introduction to air brakes” video So, we’ve got an engine driven Air compressor that supplies a reservoir which holds compressed air ready for use. The reservoir supplies the driver’s controls. Then the driver controls where the air goes. Releasing the park brake by the hand control valve supplies air to the secondary chambers compressing the power spring which takes the brakes off, and this air remains in the secondary chamber all the while the park brake is in the off position. We’re now driving along. The driver uses the foot brake pedal known as the foot control valve to control the air to the service chambers applying the brakes as and when they are needed. And once again, when we’re done, we re-apply the park brake, the air is exhausted from the secondary chambers allowing the power spring to re-assert its force and apply the brakes. Now in theory, this operation would work really well for smaller vehicles, but when we have a larger commercial vehicle, the effects of “brake lag” would make this system unusable. Let’s re-arrange the system components to where they would be on a vehicle and have a look at this “brake lag” thing. Right then, as we had before, the air compressor supplies a reservoir which holds compressed air ready for use, and the reservoir supplies
the driver’s controls. But, it takes far longer for the air to reach
the secondary chamber on the spring brake chamber and in turn compress the power spring
and release the brakes But that’s not so bad is it? It just takes a bit longer for the park brake
released. Well, let’s have a look at the service brake
then. As the driver applies the foot brake it takes a relatively long time for the air to travel to the rear service brake and build up in sufficient quantity to apply the brakes. Now if you want to stop in a hurry this is
not a good thing and could end up with serious consequences. Argh! This brake lag would also occur when the foot brake is released. So clearly, we need to find a solution to brake lag. How do we do this? By fitting a relay! Which is a device designed to speed up the application and release of the brakes. Let’s re-arrange this system with a relay for the rear service brakes and have a look at how it affects the system. Right then, what we’ve done here is put a Relay at the back, close to the rear service brakes, and also put a reservoir there, so it’s got a close by supply of compressed air. Now, we only need a signal pressure to activate the relay, so we can use small pipes, allowing the signal to get from the Foot control valve to the relay much quicker than with our previous large piping. When the driver applies the foot brake, the signal pressure is sent to the relay, which in turn uses the compressed air in its own reservoir to make that same application to the rear service brakes with the larger pipes. Minimising the Brake lag. And on releasing the foot brake, only the
signal is exhausted from the foot control valve, while the main air that we were using to apply the rear service brakes gets exhausted through the relay. And that’s how we have solved the problem of a massive brake lag…. With a relay. Now we know we can use a relay to speed up the application and release of the service brakes, we can put in relays in other systems. Like the secondary or park circuit. Right, when the driver releases the park brake, the signal pressure is sent to the relay, which in turn uses the compressed air in its own park reservoir to release the brakes, and then again, when the driver applies the park brake the signal is quickly exhausted and the park brake applied. This is all done with minimal lag. OK, so let’s have a closer look inside the
service brake relay and see how it works. Here we have a cross section of a relay, now relays come in different shapes and sizes, by different manufacturers, but they all pretty much work the same way. We have 3 main ports on the relay. The first is the supply port, which is connected directly to the reservoir so the relay has a constant supply of compressed air. The second is the delivery which goes to the rear service brake chambers, and the third is the signal port which receives the signal from the foot brake valve. And we also have an exhaust on the relay. In the rest position with the brakes off, the exhaust port is open and the delivery line is open to atmosphere, so there can be no pressure build up in the brake chambers. The relay valve is in the “OFF” position. When the driver makes a foot brake application, air is sent down the signal line and starts to act on the top of the piston. With pressure above the piston and nothing below, the piston moves down to seal off the exhaust port. With pressure still above and nothing below, the piston is forced down further until it acts on the inlet valve, which opens and allows compressed air from the reservoir to go to the brake chambers, applying the brakes. The valve is now in the “ON” position. Now, this is all happening very quickly, and what happens next is the air pressure has built up in the service brake chamber and the delivery line, and this pressure backs up and is felt on the underside of the piston. With equal pressure above and below the piston, the return spring on the inlet valve asserts itself and closes the inlet valve. With both the inlet and the exhaust valves
closed, the relay is now in the “HOLD” position as the pressure is held constant in the service brake chamber. So, the driver has his foot lightly on the brake pedal and we’ll say that the signal pressure is about 30 pounds per square inch, which means that the delivery pressure is also about 30 pounds per square inch. The drivers foot now has only 2 ways to go. Either on more, or off more. If he applies a bit more force to the brake
pedal, the signal pressure will increase. The pressure above the piston will be higher than under the piston, so the piston will move down and once again open the inlet valve, which opens and allows more air to be delivered to the brake chamber, the pressure equalises and the return spring closes the inlet valve and we once again find ourselves in the “HOLD” position. Right, this time the driver is going to remove some force from the pedal, but not all of it. The signal pressure above the piston reduces. With a higher pressure below the piston than above it, the piston is forced up, opening the exhaust valve, allowing air from the brake chambers to be exhausted through the relay. With the inlet valve closed and the exhaust open, the valve is in the “OFF” position. So when the delivery line has exhausted enough and the pressure under the piston has decreased, the pressure above acts on top of the piston, forcing it down sealing off the exhaust and places the relay once again in the “HOLD” position. Right then, the driver is now going to completely remove their foot from the brake pedal. The signal is exhausted through the foot brake valve and the air pressure under the piston from the brake chamber lifts the piston opening the exhaust valve, allowing the air to be exhausted through the relay. The valve is once again in the “OFF” position. So looking at that, the relay valve is always either in the “OFF”, the “ON” or the “HOLD” position, and all this happens very quickly. And there we have it, the air brake relay, there to minimise the brake lag and speed up the application and release of the brakes. We’ll have a look at more air brake components in other videos. Thanks for watching. Bye.

About the Author: Michael Flood


  1. I've been driving trucks for 15 years. I had a feeling there were relays used but i didn't know how they work. Now i know. I just uploaded a video show how air brakes work.

  2. Great video!!!!The simple graphics make the principles much easier to understand…..thank you very much!

  3. Thanks a lot…Excellent video. Can you do the same detailed video for the inner operation of an air compressor?

  4. Great video! Can you tell me why do some air brakes squeak when you release the brake pedal? Like the ford ones for example

  5. If I put another axle with brakes , I have to take the same signal from the pedal and putting one tank of air obiusly, great video …..

  6. This isn't the modern two line disk breaking system used on modern European articulated lorries, is it ? 
    These don't have a separate park break line do they ?

  7. very nice video cleared up the lag issues i was concerned about in air brakes as well as where the sound is coming from when air is released out the exhaust port when you let off the brake pedal as well as the brake button/throttle's on dash. Its this simple relay component that i over looked. The relay is an important components 🙂

  8. I’m a diesel mechanic in Canada and the trucks I work on use the air to release the brakes so no air means your brakes are “on” and the pedal releases the pressure in the air pot where a large spring extends a rod that pushes on a ratchet and rotates the
    s-cam rather quickly.

    Does this system not exist elsewhere?

  9. Hey guys, I have a question regarding truck transmissions. Sometimes when trucks drive through deep flood waters, it is said that this will/can destroy the transmission or differentials, as they are ventilated and water may get in – is there anything to that claim or is it BS?

  10. I have a 2017 Frieghtliner Cascadia and the Air Dryer is located right in front of the passenger tire off the chasis, very easy to get at !!!

  11. Great video, but as a diesel mech in the works, ive never seen a air tank any closer to the cams than the primary tank. I dont think the relay has anything to do with timing of air pressure but the simple fact of it having an exhaust. Abs ties into this relay to regulate the brake pressure when turning or to prevent lock up.

  12. lol, I'm an instrument tech looking for information on pneumatic relays and this was the best video I could find to explain it. well done

  13. Great explanation. Well done. I have one common question. What is use of solenoid valve in addition with relay valve and how its functioning?

  14. Its a pretty good video i know for dumies it would be nice to explain how is the air going from the supoly through the bolt/inlet valve into the brake delivery

  15. I understood that it is the other way around, when air pressure is in the chamber is when the brake is released.and when you take the air away the brake is apply.

  16. How to we strong air break truck .. . I have a truck car that we repair all of things to changed new . But don't have any break to stop the car what we do about this truck . Please aware to me.. thanks..

  17. If u have good drums new slacks new brakes . But your can rotation gauge says your still over stroking what's wrong

  18. This was just awesome! I LOVED the "tu tu du…" part. Reminded me of one of the most epic stand up comedians I know, Eddie Izzard!

  19. Hi !
    Can you help me with this question plz ?

    What must the driver of a heavy véhicule equipped with air brakes keep in mind about the response time for the air brake système as compared to a véhicule equipped with a hydraulic brake système ?


  20. Very nicely explained & the sound you make in between daa daaa makes me laugh. Keep it up. Thanks for the video.

  21. This satisfies my curiosity of that air below vehicle blown on the road. Exhaust… Air released from delivery pipe line.

    Thank you.

Leave a Reply

Your email address will not be published. Required fields are marked *