How Do Semi Truck Air Brakes Work?

When the brakes are applied, circuits send compressed air from the storage tanks to front and rear service chambers that push the brake shoes into the brake drums, thus slowing down the vehicle. Semis equipped with air disc brakes work on a similar principle, but use pads and discs instead of shoes and drums. As many big rig reviewers have pointed out, they unlock improved braking performance and reduce maintenance. Still, although innovation makes trucking easier, that doesn't mean anyone can drive a semi. Try backing up an 18-wheeler – that's no small feat, shall we say. 

That, of course, was a quick and dirty explanation of how air brakes work. But you may be thinking: what even are service chambers? How does the system work when there's an air leak? Are there checks and balances in place should something go wrong? Before we jump into all of that, let's get familiar with the components inside a semi's air brake assembly.

It starts with the pedal itself (also called the foot valve), followed by the air compressor, governor, storage/reservoir tanks, dryer, drain valves, brake chambers, S-cams, shoes/pads, drums/discs, pushrods, pressure plates, and return springs. If you're still with us, let's now dive into brake chambers and understand their role in the system.

After passing through the dryer, compressed air is sent to the supply tank (the main source). From there, it flows into the primary reservoir (for the rear brakes) and the secondary reservoir (for the front brakes) in a dual-circuit brake system. The governor prevents the compressor from exceeding the safe pressure limits. When the brake pedal is depressed, the brake valve sends the compressed air into both the primary and secondary circuits, which feed their respective service brake chambers.

Trucks typically have single-chamber units at the front axle and piggyback brake chambers (also known as combination/spring brake chambers) at the rear, with the latter doubling as the truck's parking brake and the emergency brake when there's a sudden pressure drop. Inside the front brake chamber, the compressed air pushes against a diaphragm, forcing the pushrod inside to actuate the S-cam (via slack adjusters), which presses the brake shoes against the brake drum — semis still use drum brakes. When the driver releases the brakes, a return spring brings the pushrod back to its original position.

The rear spring chamber, however, is designed differently. Normally, it operates like a single service chamber. But it's also set up to act as a fail-safe when there's a leak. The chamber's piggyback side contains an extra-strong spring (capable of 2,000 lb of force) that remains compressed until there's a pressure drop, which releases the spring, thus engaging the brakes. Put simply, the piggyback is designed so that air pressure keeps the brakes disengaged, while the absence of air pressure applies them, which explains why it also doubles as the truck's parking brake.