Brakes operate on friction. Brake pads meet the drum (or metal disc on disc brakes). When these meet friction occurs, which in turn causes the wheels (attached to the brakes) to slow. The linings and/or pads are held against the drum/disc with either hydraulic, vacuum or air pressure.
Hydraulic brakes use fluid pressure to control brakes. Lines hold the hydraulic fluid between the brake pedal and brakes, with cylinders at both ends. The master cylinder puts pressure on the fluid, which moves to the wheel cylinder. The pressure pushes on the wheel cylinder, which moves the brake shoe and pad into contact with the drum/disc.
Vacuum brakes have a cylinder with a movable piston. The pressure of the atmosphere trying to fill the vacuum pushes the piston into the vacuum. The vacuum brakes works in concert with hydraulic brakes.
Nearly every CMV manufactured after March 7, 1990 is required to have parking brakes. The parking brake control can be a lever or a knob, which is easy to use. Some parking brakes are operated on a cable system. Other systems use air pressure. This is a separate air supply from that used for the service brakes. Because you must be able to immediately reapply parking brakes, air pressure cannot be used to keep the parking brakes set.
Service brakes must still work on a tractor even if the trailer breaks away. Also, there must be two ways to employ emergency brakes. One requires they apply automatically if the psi on your air supply falls to 20 to 45 psi. The other one requires the emergency brakes be able to be applied manually. The air system for the emergency brakes must be separate from all other truck controls.
The driver must be able to reach the emergency brakes while seat belted into the driver's seat. Not all three brakes may be combined into one system, although the emergency brakes may be combined with either the service or the parking brake. The FMCSA requires that no matter the situation at least one of the brakes must always work.
Brake Tubing and Hosing
CMV drivers must inspect the brake lines for leaks, cracks or other problems.
Largely all brakes must work at all times.
The brake reservoirs can have no leaks. Drivers need to make sure the check valve is intact. The engine needs to be turned off in order to properly test the check valve.
Warning Devices and Gauges
Your CMV must have visual and audio alerts on low pressure. These warning devices must always work.
In order to test brake performance, a driver needs to check braking speed and distance. There are three performance measures. 1) Speed and distance requirements assume a traveling pace of 20 miles per hour. 2) The braking rate in feet per second per second. 3) Braking distance is how far the vehicle travels before it stops.
The requirements of CMV brake operations are found under the FMCSR Part 393.
All wheels on a CMV must have working brakes.