I85PREP
⌘ K
MT CDL Air Brakes study materials
MT CDL Air Brakes study resources

MT CDL Air Brakes: Air Brake System Components and Their Functions

MT CDL Air Brakes · Work through your exam, one topic at a time.

All guide sections

Air Brake System Components and Their Functions

This domain tests whether you can name each component of a vehicle air brake system, state the pressure at which it acts, and explain how it interacts with the rest of the system. Montana questions tend to be device-specific: which part does what, and what a given symptom points to.

Air supply and storage

  • The compressor is engine-driven (belt or gear) and charges the system through an air dryer that removes moisture and oil vapor.
  • The governor senses reservoir pressure and cuts the compressor out at roughly 125 psi, cutting it back in near 100 psi.
  • The safety valve, mounted on the wet (supply) tank, vents at about 150 psi to protect the system if the governor fails.
  • The wet tank receives air first and collects most of the water; drain cocks on every tank must be opened daily.
  • Check valves allow air to flow one way only, so a leak in one circuit cannot bleed the other.

Control and delivery

  • The foot (treadle) valve meters service air to the brakes according to pedal travel.
  • Relay valves on trailers speed up fill and release; quick-release valves exhaust air fast so brakes release promptly.
  • Glad hands and the supply/service lines connect the trailer; the tractor protection valve guards the tractor's air if the trailer separates.

Foundation brakes

Air pressure pushes the brake chamber diaphragm, moving the push rod, which turns the slack adjuster and the S-cam to spread the shoes against the drum. Disc and wedge designs use the same air logic.

Worked example

A tractor loses air only while the pedal is held down; the released-brake leak test passes. Reasoning: the supply side is tight, so the loss is downstream of the foot valve in the service side - a chamber diaphragm, hose, or fitting. That is an applied-pressure leak, not a compressor problem.

Traps that cost marks

Confusing the safety valve with the governor; assuming a slow build rate means a bad gauge; forgetting that the wet tank holds the most water; thinking the alcohol evaporator and the air dryer do the same job.