How to do a Leak Test for CDL Air Brakes

Applying the service brakes and doing a leak test will often locate a ripped service brake diaphragm.

How to do a Leak Test for CDL Air Brakes | Air Brake Smart


Closed Caption


Hi there smart drivers, Rick with Smart Drive Test - quick tip for trucking smart.

Your leak test, which you not only have to do as part of your daily pre-trip inspection.

For new drivers you also have to do it as part of your license test.

And for a leak test, after you do all of the other in-cab checks on your air brake system, you're allowed to lose three pounds after the initial drop after you apply the service brakes.

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Safety First

So you have your wheels chocked and your brakes--your parking brakes--released.

You make a full service brake application - after the initial drop, on a single unit you're allowed to lose three pounds, on a tractor-trailer you're allowed to lose four pounds, and on a truck with two trailers or multiple trailers you are allowed to lose six pounds in one minute.

So you have to do a leak test.

Now on this truck - this truck is a year old.

It's not a very old truck, it only has about 8,000 kilometers (4,900 miles) on it, so we just did a brake adjustment and we put the service brakes on.

A torn service brake diaphragm can be quickly swapped out by a mechanic!

Release the Parking Brakes

The brakes are released and one of the reasons that you release the parking brakes and chock the wheels and have over 90 pounds of pressure in the system is to detect air leaks.

If you have air leaks on the system you obviously know there's something wrong.

So it has to be repaired and if you lose more than those allowable limits it has to go into a mechanic and be checked because it's going to be out of service and you're not going to be able to operate the vehicle.

No Leak When All Brakes are Released

So you can see right now I have this the parking brakes released and there's no air leaks.

And so one of the things you're doing when you're going around on your pre-trip and you have your wheels chocked--the parking brakes released with over 90 pounds--you're always listening for air leaks because there's a lot of air in the system.

And that's what you're listening for.

So right now the parking brakes are released and there's no air leak, so what I'm going to do is I'm going to apply the service brakes and you'll hear that there's an air leak on this truck.


Leak When the Service Brakes are Applied

So as soon as I put the service brakes on and apply the service brakes...I have a stick holding the brake pedal on and you can hear up here on the firewall.

There is a leak in the air brake system - it's not a very big leak, but you can hear that - you could probably even hear that over the diesel engine.

And I suspect because it's up here...

now remember when you're in a car you're sitting down here [HAND INDICATING LOW TO THE GROUND], but when you're in a big truck, you're sitting up here [HAND SHOWING 5' OFF GROUND] this is probably the brake pedal valve that's probably leaking a little bit there.

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Get It Repaired

And when I take it back--it's not a huge leak; it's a minor leak--but I'll take it back and I'll get them to check it.

So leak test on a big truck - you need to do a leak test.

Most of the time what a leak test is going to tell you is that you have a ripped diaphragm in one of the service brake chambers and a mechanic can switch those out pretty quickly.

Or one of the valve is leaking as you can see here.

So a leak test, you have to do it as part of your pre-trip inspection every day; for new drivers taking a license, you have to do it as part-and-parcel of your in cab pre- checks on the airbrake system.

Question for my smart drivers:

Have you ever found a ripped diaphragm on your vehicle by doing a leak test?

Leave a comment down in the comment section there, all that help us out.


I'm Rick with Smart Drive Test.

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Good luck on your license and remember pick the best answer not necessarily the right answer.

Have a great day.

Bye now.

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