Jason the mechanic: Air brakes and the simple explanation

When we get in our semis and straight trucks to haul grain, we are glad when we hit the brake peddle to stop and we stop. The air brake system on our trucks is actually pretty simple and works the same on almost all trucks.

All air brake systems use air pressure to apply the brakes when you step on the pedal. The air is stored in a series of pressure tanks on the truck. The air is pressurized by the air compressor on the

truck’s engine. The pressure is regulated by the air governor on the air compressor. Most, if not all systems, work on 120 psi of pressure.

Some systems have an air dryer, which dries the moisture out of the air to keep it from freezing in the winter time.

The air dryer has a cartridge in it that should be changed once per year. The air is pressurized by the compressor,

passes through the air dryer and into the tanks. From the tanks, it goes to two different systems.

The first system is the emergency system. This is the system that releases the parking brake and holds some reserve air to stop you if there is a leak. The way the parking brake system works on most trucks is there are springs in the air chambers on the rear axle or axles of the truck that apply the parking brakes. When you

push the parking brake knob in, this supplies air to those air chambers and pushes against the springs to release the parking brakes. If the pressure in the air system pulls below 60 psi the parking brake knob will pop out and set the parking brakes.

This is a safety feature so the truck will stop, because if the pressure falls too low, the service brakes will not work. If you are driving a semi tractor with a trailer, the trailer knob for the brakes works in the same way. But if there is an air leak in the trailer, the tractor protection valve will make the trailer brakes set to protect the tractor’s air supply so you can still stop.

The other system is the service brake system. This is the system that stops you when you push on the brake pedal. When you push on the brake pedal it goes through the pedal valve, which modulates the air flow to the brake chambers. The brake chambers in turn push on the slack adjusters, which is the component that keeps the brakes in adjustment. There are different types of slack adjusters — manual and automatic.

Manual slack adjusters are just as they sound. You must manually adjust them to keep the brakes in adjustment. These are, in my opinion, the best style to have.

The automatic slack adjusters automatically adjust themselves to keep the brakes in adjustment. These work well if you keep them greased. The problem is that most people don’t keep them greased and they seize up and don’t work. If you do have automatics, the best grease to use on them is white lithium grease.

The slack adjusters turn a shaft that turns the S cams, which push the brake shoes out into the drums. The S cam shafts have bushings in them that should be checked every time you change the shoes. They are also grease-able.

There are such things as air disc brakes but they are not very common. Most brake shoes have an indicator built into the end of the pads that tell when they need to be replaced. When you replace the brake shoes you should also get hardware kits that include the springs, pins and bushings for the brakes. You should also always replace the drums when you replace the brake shoes. This is a good idea because the drums will have a wear ridge on them and could also have heat cracks in them.

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198 comments

  1. i am having volvo fh 2007 i have repalced the 4 way valve but still the air is full in the vehcile but the stop sign comes and parking brake doesnot get relaeased may i know what is the reason for that

  2. i have a 2006 volvo truck d 12 air pressure will not build up

  3. I have an 08 Gu713 Mack, I have replaced the treadle valve and double checked all 6 slack adjusters to be working correctly. My question is when I apply my brakes they are very soft and when I’m loaded they will barely stop me. What else could it be?

  4. Hello I have a 05 Freightliner FLD day cab. Air leaks inside cab only when foot valve is pressed. replaced foot valve checked all chambers and still have the problem. The brakes work good but the air is leaking behind the foot valve where it bolts to firewall. Any ideas would be great i have tried everything I know. every thing i read on trouble shooting consisted of leaks out exhaust and this one does not. Thanks

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