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BC Class 1–4 overview

BC Class 1–4: Air brake components

Why air brakes, brake chambers and slack adjusters, s-cam brakes, compressor, governor, reservoirs, foot valve, air dryers and ABS.

  • 18 questions
  • Topic drill
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Air brake components · Question 1Score 0/0

How do you get more braking from the foot valve?

Study reference: Driving Commercial Vehicles (ICBC, 2024 edition), Ch. 8 Air brakes — Basic components, Compressor, Governor, Reservoirs, Foot valve, Air dryers, Anti-lock braking systems

All 18 questions in this test

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

  1. How do you get more braking from the foot valve?

    1. Press the pedal farther
    2. Press harder without moving it
    3. Pump it quickly
    4. Hold it halfway and wait

  2. With an air dryer fitted, should you still drain the reservoirs?

    1. Only if the dryer fails
    2. Only in summer
    3. Yes, regularly
    4. No, never

  3. What does the governor do when system pressure reaches cut-out?

    1. Applies the spring brakes
    2. Unloads the compressor
    3. Opens the safety valve
    4. Shuts off the engine to save fuel

  4. At fast idle, how quickly must the compressor build pressure from 50 to 90 p.s.i.?

    1. Within 30 seconds
    2. Within three minutes
    3. Within one minute
    4. Within ten minutes

  5. How can you identify a long stroke brake chamber?

    1. A red painted housing
    2. A round tag on the slack adjuster
    3. Square inlet ports or a trapezoid-shaped tag
    4. A larger return spring visible outside

  6. A compressor belt deflects more than double its width when pressed between pulleys. What does this mean?

    1. It is too tight
    2. It needs adjusting
    3. It is correctly tight
    4. It must be replaced only if cracked

  7. In an s-cam brake, what spreads the linings against the drum?

    1. The s-shaped head of the camshaft
    2. A wedge between rollers
    3. The brake shoe return springs
    4. Hydraulic pistons

  8. About how much pushrod force does a Type 30 brake chamber give at 100 p.s.i.?

    1. About 30,000 pounds
    2. About 300 pounds
    3. About 1,000 pounds
    4. About 3,000 pounds

  9. At about what pressure does the safety valve on the first reservoir open?

    1. About 100 p.s.i.
    2. About 150 p.s.i.
    3. About 125 p.s.i.
    4. About 60 p.s.i.

  10. Why can an air brake system tolerate a small leak better than a hydraulic one?

    1. Air brakes have no lines to leak
    2. Hydraulic fluid evaporates faster
    3. Air cannot escape through small holes
    4. The compressor keeps making more air

  11. How much application pressure does a normal stop usually use?

    1. About 60 p.s.i.
    2. About 100 p.s.i.
    3. Less than 20 p.s.i.
    4. Full system pressure

  12. Why is the supply reservoir called the wet tank?

    1. It holds the alcohol anti-freeze
    2. Water and oil sludge collect in it
    3. It is mounted under the fuel tank
    4. It is filled with brake fluid

  13. The ABS warning light stays on while you drive. What does this mean?

    1. All braking has failed and you must stop now
    2. The air pressure is too high
    3. The parking brakes are applied
    4. ABS has failed, but normal brakes still work

  14. Your reservoirs hold 80 p.s.i. What is the most application pressure you can get?

    1. 80 p.s.i.
    2. 160 p.s.i.
    3. 100 p.s.i.
    4. 120 p.s.i.

  15. Cut-out is set at 125 p.s.i. At about what pressure should the governor cut in?

    1. About 85 p.s.i.
    2. About 60 p.s.i.
    3. About 105 p.s.i.
    4. About 115 p.s.i.

  16. When does the air compressor run?

    1. Only when the brakes are applied
    2. Only above 60 p.s.i.
    3. Whenever the engine is running
    4. Only when the driver switches it on

  17. Where is an air dryer installed?

    1. On the trailer glad hands
    2. Inside the brake drum
    3. Between the compressor and the first reservoir
    4. Between the foot valve and the chambers

  18. Which part both adjusts for lining wear and acts as a lever to multiply force?

    1. The relay valve
    2. The foot valve
    3. The governor
    4. The slack adjuster

Air brake components: 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 ↗

Why air brakes? (p. 145–146)

  • Air brakes apply much more force than hydraulic brakes, which heavy commercial loads need.
  • Air brakes tolerate small leaks because the compressor makes more air as needed; a small leak in a hydraulic system could mean brake failure.
  • Compressing air into a smaller space creates pressure that can be turned into mechanical force.
  • 10 pounds of force on one square inch is 10 p.s.i., about 68.9 kPa.

Basic components and force multipliers (p. 146–149)

  • The simplest air brake system has a compressor with a governor, air lines, a reservoir, a foot valve (brake pedal) and foundation brakes.
  • Foundation brakes include brake chambers, slack adjusters, brake linings and drums or rotors.
  • The slack adjuster both adjusts for lining wear and acts as a lever that multiplies force.
  • The brake chamber turns air pressure into mechanical force through the pushrod and slack adjuster.
  • A brake chamber has a flexible diaphragm (reinforced fabric with a rubber coating) clamped between two steel housings, a pushrod and plate assembly and a return spring.
  • Pushrod force depends on air pressure (p.s.i.) times the effective diaphragm area (square inches).
  • A Type 30 chamber has 30 square inches of effective area and gives about 3,000 pounds of pushrod force at 100 p.s.i. (690 kPa).
  • Chambers range from Type 9 (9 square inches) to Type 36 (36 square inches) so each axle gets the right braking force.
  • The Type 30 clamp chamber is the most common size on truck drive axles and trailer axles.
  • Long stroke chambers have a longer pushrod stroke and are identified by square inlet ports and/or a trapezoid-shaped tag on a clamp bolt.
  • Normal stops usually use less than 20 p.s.i. (138 kPa) of application pressure even though system pressure is 100 p.s.i. or more.
  • With air applied, the slack adjuster sits at about 90 degrees to the pushrod.

Foundation brakes: s-cam type (p. 149–150)

  • The foundation brake is the brake assembly at each wheel, including the brake chamber; the s-cam drum brake is the most popular type.
  • Air pushes the pushrod forward, the slack adjuster rotates the camshaft and its s-shaped head spreads the linings against the drum.
  • Brake shoe return springs pull the linings away from the drum when the air is released.
  • The linings are mounted on the axle housing; the drum is mounted on the wheel and turns with it.

Compressor (p. 150–152)

  • The compressor takes in outside air, compresses it and pumps it to the supply reservoir.
  • It is driven by the engine, by gears or by a belt, and runs whenever the engine runs.
  • All trucks use piston-type compressors with one, two or four cylinders.
  • At 120 p.s.i. (827 kPa), air leaving the compressor is over 204 °C (400 °F).
  • Most heavy-truck compressors are cooled by engine coolant; some on lighter units are air-cooled.
  • The compressor is usually lubricated by engine oil, sometimes by its own supply; oil also helps cool it.
  • The compressor shares the engine's air filter, which must be checked so it is not clogged.
  • Intake stroke: piston moves down, inlet valve open, air drawn in; compression stroke: inlet valve closed, discharge valve opens near the top and air goes to the reservoir.
  • Check a compressor belt by pressing midway between pulleys; if it deflects more than double the belt's width, it needs adjusting.

Governor (p. 152)

  • A compressor can make over 500 p.s.i. (3,448 kPa); most systems run at a maximum of about 125 p.s.i. (862 kPa).
  • The governor "unloads" the compressor (stops it pumping) at about 125 p.s.i. by making the unloader piston hold the inlet valve open.
  • Maximum (cut-out) pressure may range from 105 to 135 p.s.i. (724 to 931 kPa); the spread between cut-in and cut-out should be about 20 p.s.i. (138 kPa).
  • Example: cut-out at 125 p.s.i. means cut-in at about 105 p.s.i.
  • Safety standards require the governor to restart the compressor if pressure falls below 80 p.s.i. (552 kPa); in any case it must cut in by 80 to 85 p.s.i.
  • The compressor must build reservoir pressure from 50 to 90 p.s.i. within three minutes at fast idle (1,000–1,200 r.p.m.).

Reservoirs (p. 153)

  • Steel reservoirs store compressed air.
  • A safety valve (pop-off valve) on the first reservoir is a spring-loaded ball that vents excess air, normally at about 150 p.s.i. (1,034 kPa), if the governor fails to unload the compressor.
  • If the safety valve has to vent, the governor or compressor needs repair by a qualified mechanic.
  • Compressed air carries water vapour, oil and carbon that form a grey sludge; the supply reservoir is therefore called the wet tank.
  • Water causes valve trouble and can freeze in winter, jamming valves or brake chambers.
  • Every reservoir has drain valves (drain cocks); most makers recommend draining daily and letting the air drain completely.
  • Stay out of the path of escaping air when draining; it can injure eyes and ears.
  • Some reservoirs have more than one compartment and more than one drain cock.

Foot valve and how air brakes work (p. 153–155)

  • The foot valve (treadle) is a foot-controlled pressure regulator; pressing it farther, not harder, gives more braking.
  • Holding the treadle still holds the application pressure constant, and the valve tops up small leaks downstream automatically.
  • Releasing the treadle exhausts application air through the foot valve's exhaust port.
  • A brake application can never exceed reservoir pressure; with 80 p.s.i. in the reservoirs, the maximum application is 80 p.s.i.
  • On release, chamber return springs pull the pushrods back and shoe return springs pull the linings off the drums.

Other types of foundation brakes (p. 177–180)

  • Besides s-cam brakes, air-braked vehicles may have wedge brakes, air disc brakes or air-over-hydraulic brakes.
  • Wedge brakes use one or two small chambers with wedge-shaped pushrods that push between rollers to spread the linings; they are usually only on steering axles.
  • Most wedge brakes have internal automatic adjusters; if either lining moves more than 1/16 inch, or both together more than 1/8 inch, the adjusters have failed.
  • Drivers cannot easily check wedge or air disc brake adjustment; only a qualified mechanic should adjust or repair them.
  • Air disc brakes squeeze two pads against a rotor on the hub, like a large C-clamp; most self-adjust and have the same chamber stroke limits as automatic slack adjusters.
  • Air-over-hydraulic brakes are common on middleweight trucks and buses: air actuates hydraulic pressure intensifiers, and hydraulic pressure applies the foundation brakes.
  • Air-over-hydraulic systems have a compressor, governor, tanks, foot valve and two intensifiers, run at about 125 p.s.i., and may have spring parking brakes that force a wedge between the shoes.
  • Diesel engines do not provide usable vacuum for brake boosters, which is why a partial air system is used on these vehicles.
  • An air brake endorsement is required for most air-over-hydraulic vehicles; the only exception is a system without an air treadle.

Air dryers, alcohol systems and automatic drains (p. 180–182)

  • An air dryer sits in the discharge line between the compressor and the first reservoir and removes water vapour, oil mist and carbon.
  • When the reservoir reaches full pressure, the dryer's purge port opens and blows out the collected contaminants; clean air back-flushes the filters.
  • Some air dryers have an electric heater to prevent freezing; with a dryer, the safety valve is often mounted on the dryer.
  • Even with an air dryer, drain the reservoirs regularly; more than a few drops of water may mean the dryer or compressor needs service.
  • Integrated dryer systems combine filter, dryer and governor with a purge reservoir and no wet tank; one service reservoir fills first, the other starts at about 100 p.s.i., and maximum may be up to 135 p.s.i.
  • Alcohol evaporators (on the compressor intake side) and alcohol injectors (in the discharge line, using a venturi) add alcohol vapour as anti-freeze for moisture.
  • Keep the alcohol reservoir topped up with methyl hydrate made for air brake systems through winter, starting before the first freeze; reservoirs must still be drained.
  • Automatic drain ("spitter") valves expel contaminants each time reservoir pressure drops 2 or 3 p.s.i., or each time the compressor cycles; open the manual drains periodically anyway.

Front wheel limiting, emergency release, pressure protection (p. 182–184)

  • An automatic front wheel limiting (ratio) valve has no dash control; below about 5 to 15 p.s.i. no air reaches the steering axle brakes.
  • Below 40 p.s.i. application the automatic limiting valve gives the steering axle about 50 per cent of drive axle pressure; at emergency pressures of 60 to 70 p.s.i. both axles get equal pressure.
  • Manual front wheel limiting valves (no longer installed on new vehicles) have a dash control: "dry" gives equal pressure, "slippery" gives the steering axle 50 per cent.
  • The slippery-road position may be used only when weather makes it essential for safety; front wheel skids are less dangerous than drive axle lockup.
  • A spring parking brake emergency release system ("California" system) has a separate tank and a hold-down ("dead man") dash valve to release spring brakes to move a disabled vehicle; it is still found on some transit buses, school buses and fire trucks.
  • Pressure-protection valves cut air to non-essential accessories (air seats, horns, wipers, suspension, fifth wheel sliders, air shifts) if a failure drops service pressure below a preset point of 60 to 90 p.s.i.
  • Application pressure gauges show the actual pressure delivered to the service brakes, sometimes separately for tractor and trailer.

Anti-lock braking systems (p. 184–186)

  • ABS has three parts: wheel speed sensing, decision-making by an electronic control unit (ECU), and brake release or modulation.
  • Magnetic pickups near toothed rings on the hubs generate a pulsing current; a sudden drop tells the ECU a wheel is locking.
  • Solenoid valves called modulators release and reapply air at the affected wheel, up to about five times per second.
  • On a very slippery surface, apply and hold firm, continuous pedal pressure and let ABS work; noise and vibration are normal and you keep steering control.
  • The ABS warning lamp comes on at ignition for a self-check; if it stays on or lights while driving, ABS has failed but normal braking still works and the vehicle may be driven to a service depot.
  • Trailer ABS has a warning light on the left side of the trailer (front or rear) visible in the tractor mirror; some tractors also have a trailer ABS dash light.
  • The trailer ABS ECU may be powered from the stop lamp circuit or a dedicated circuit in the electrical connector.

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