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Saskatchewan Commercial overview

Saskatchewan Commercial: Air brake components and system operation

Compressor and governor pressures, reservoirs and draining, foot valve, brake chambers, S-cam brakes and disc brakes, safety valve, check valves, gauges, relay and quick-release valves, low air warning, ratio valve and air dryer.

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Air brake components and system operation · Question 1Score 0/0

What does a front axle ratio valve do during light brake applications?

Study reference: SGI Air Brake Manual (2025-26), Ch. 3 Basic system components and Ch. 4 Basic system operation

All 14 questions in this test

Every question and its answer choices. The correct answers and explanations appear when you take the test above.

  1. What does a front axle ratio valve do during light brake applications?

    1. Reduces pressure to the front brakes
    2. Doubles the application pressure to the front brakes
    3. Shuts off the rear brakes
    4. Applies only the trailer brakes

  2. At what pressure must the governor cut out the compressor?

    1. Between 80 and 100 psi
    2. Between 60 and 70 psi
    3. Exactly 150 psi
    4. Between 120 and 145 psi

  3. The safety valve on the supply reservoir blows off air. What does this indicate?

    1. A problem with the governor
    2. A worn brake lining
    3. Normal operation in cold weather
    4. A leak in the service line

  4. Your truck has an air dryer. Do you still need to drain the reservoirs?

    1. Only if the air dryer's own warning light comes on
    2. Only once a month during the summer driving season
    3. Yes, daily; the dryer does not remove all moisture
    4. No, the dryer removes all the water

  5. What does a quick-release valve do?

    1. Opens the safety valve when pressure is high
    2. Applies the brakes faster in an emergency
    3. Exhausts air near the chamber to release brakes faster
    4. Lets the driver release the spring brakes

  6. What is the job of the air compressor?

    1. To build up and maintain air pressure
    2. To remove water from the air supply
    3. To apply air to the brake chambers
    4. To store air for several brake stops

  7. Why is the first reservoir also called the wet tank?

    1. It sits closest to the fuel tank
    2. It is the only tank fitted with a draincock
    3. It is filled with antifreeze in the winter
    4. Most water and oil from the compressor collect there

  8. Why is a relay valve installed near the rear brake chambers of a long vehicle?

    1. To reduce brake lag at the rear wheels
    2. To lower air pressure to the front brakes
    3. To hold the brakes on when parked
    4. To dry the air before it reaches the brakes

  9. At what pressure must the low air warning activate?

    1. Below 100 psi (690 kPa)
    2. Below 45 psi (310 kPa)
    3. Below 20 psi (138 kPa)
    4. Below 60 psi (414 kPa)

  10. Why are the front brake chamber and slack adjuster mounted on the backing plate?

    1. Because it keeps them away from road heat
    2. Because the front axle has no S-cam
    3. Because the front brakes need more air
    4. Because the axle's steering action would interfere

  11. What is the minimum pressure at which the governor should cut the compressor back in?

    1. 80 psi (552 kPa)
    2. 120 psi (828 kPa)
    3. 60 psi (414 kPa)
    4. 100 psi (690 kPa)

  12. How often must air reservoirs be completely drained when the vehicle is in use?

    1. Only when the air dryer fails
    2. Once a week
    3. Once a day
    4. Once a month

  13. What is the purpose of the one-way check valve between the supply and service reservoirs?

    1. It warns the driver of low pressure
    2. It stops air flowing back if the supply side fails
    3. It lets air flow both ways to balance tanks
    4. It releases excess pressure into the air

  14. What limits the maximum pressure you can apply to the brakes?

    1. The strength of your leg
    2. The air pressure in the reservoir
    3. The size of the foot valve
    4. The governor cut-in setting

Air brake components and system operation: key rules to remember

Our summary of what the official handbook says on this topic, in our own words. Read it before or after the practice questions above, then check the original chapter. Read the official chapter ↗

Ch. 3 Basic system components (pp. 8–11)

  • Five basic components: compressor (builds and keeps pressure), reservoirs (store air), foot valve (draws air for braking), brake chambers (turn air into mechanical force), shoes and drums or rotors and pads (make friction).
  • Compressors are gear or belt driven, run whenever the engine runs, and are loaded or unloaded by the governor; self-lubricated compressors need their oil checked and changed.
  • The governor cuts out at 120–145 psi (828–1,000 kPa) and cuts in at no less than 100 psi (690 kPa), normally 20–25 psi below cut-out.
  • Reservoirs must hold enough air for several applications if the engine stops; you cannot apply more pressure than is in the reservoir.
  • Every reservoir has a draincock; drain all reservoirs completely once a day when in use.
  • The foot (treadle) valve delivers more air the farther it is pressed; releasing it exhausts air from the service brakes.
  • A service brake chamber's diaphragm pushes the push rod when air enters; a return spring pulls it back when air is released.
  • S-cam brakes: the slack adjuster turns the cam, the rollers force the shoes against the drum; front brakes have chamber and slack adjuster on the backing plate because of steering.
  • Thin or distorted drums, weak return springs, wrong linings, poor adjustment or grease on linings make braking erratic.
  • Air disc brakes have an internal automatic adjuster inside a sealed unit.

Basic system operation and additions (pp. 12–18)

  • Supply (wet) reservoir: first tank from the compressor, where most water and oil collect; service reservoir supplies the brakes.
  • Safety valve: protects the supply reservoir, usually set at 150 psi; if it blows, the governor is faulty and needs a qualified person.
  • One-way check valve between supply and service reservoirs stops air flowing back if the compressor or supply tank fails.
  • The dash air pressure gauge is plumbed after the service reservoir.
  • Relay valve near the rear chambers reduces brake lag on long vehicles; the foot valve line becomes a control line.
  • Low air warning: switch after the supply reservoir, activates below 60 psi (414 kPa); the light is mandatory, the buzzer optional; stop and find the cause.
  • Stop light switch: air-operated electrical switch that lights the brake lights on any application.
  • Quick-release valve: exhausts air near the chamber so brakes release faster.
  • Front axle ratio valve: reduces front brake pressure on light applications, full pressure from about 60 psi; older trucks may have a dash switch cutting front pressure by 50%.
  • Air dryer: desiccant filter removing most moisture before the reservoirs, helps prevent freeze-ups; reservoirs must still be drained daily.

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