How Do Air Brakes Work for Semi Trucks? – Jalopnik

When the brakes are applied, circuits send compressed air from the storage tanks to the front and rear service chambers that push the brake shoes into the brake drums, slowing the vehicle. Semis equipped with air disc brakes work on a similar principle, but use brake pads and discs instead of shoes and drums. As many reviewers of large rigs have noted, they provide improved braking performance and reduce maintenance. However, while innovation makes trucking easier, that doesn’t mean everyone can drive a semi. Try reversing an eighteen-wheeler – no small feat, let’s say.

Of course, that was a quick and dirty explanation of how air brakes work. But you may be thinking: what are service rooms anyway? How does the system work in the event of an air leak? Are there checks and balances in place if something goes wrong? Before we get into all that, let’s first get familiar with the components in a semi’s air brake assembly.

It starts with the pedal itself (also called the foot valve), followed by the air compressor, regulator, 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, now let’s dive into the brake chambers and understand their role in the system.

After the compressed air passes through the dryer, it 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 braking system. The regulator prevents the compressor from exceeding safe pressure limits. When the brake pedal is depressed, the brake valve sends the compressed air to 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, the latter also serving as the truck’s parking brake and as an emergency brake in the event of a sudden drop in pressure. In the front brake chamber, the compressed air pushes against a diaphragm, forcing the pushrod inward to operate the S-cam (via slack adjusters), which presses the brake shoes against the brake drum – semi-finished still use drum brakes. When the driver releases the brakes, a return spring returns the push rod to its original position.

However, the rear spring chamber is designed differently. Normally it operates as a single service room. But it is also set up in such a way that it can act as a fail-safe if there is a leak. The piggyback side of the chamber contains an extra strong spring (capable of 2,000 lb of force) that remains compressed until a pressure drop occurs, releasing the spring and thus applying the brakes. Simply put, 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.