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Saskatchewan Commercial: Brake adjustment and air brake pre-trip tests

Why poor adjustment causes brake failure, push rod stroke limits, manual and automatic slack adjusters, the hammer test, service tests, steep downgrade checks, and pre-trip air tests for single and combination units.

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Brake adjustment and air brake pre-trip tests · Question 1Score 0/0

After adjusting, you tap the brake drum with a hammer and hear a dull sound. What does it mean?

Study reference: SGI Air Brake Manual (2025-26), Ch. 7 Checking and adjusting cam-type brakes — Brake adjustment, Slack adjusters, Pre-trip procedures

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. After adjusting, you tap the brake drum with a hammer and hear a dull sound. What does it mean?

    1. The brake is perfectly adjusted
    2. The lining is new and still bedding in
    3. The brake is dragging and needs readjusting
    4. The drum is cracked and must be replaced

  2. Why might badly adjusted brakes go unnoticed?

    1. They only fail in warm weather
    2. The low air warning hides them
    3. They make the pedal feel harder
    4. They seem fine under light braking

  3. After adjusting a manual slack adjuster, how much push rod travel should there be when pulled by hand?

    1. Less than 1/8 inch
    2. 1 to 1 1/2 inches
    3. 1/2 to 3/4 inch
    4. About 2 inches

  4. How can heat from hard braking reduce reserve chamber stroke?

    1. Hot cast-iron drums expand, adding push rod stroke
    2. Hot air in the chamber shrinks the diaphragm
    3. Heat stiffens the return spring
    4. Heat raises the governor cut-out pressure

  5. How often must automatic slack adjusters be checked?

    1. Weekly
    2. Only at the PMVI inspection
    3. Daily
    4. Monthly

  6. In a governor test, at what pressure should the compressor cut out?

    1. Between 150 and 170 psi
    2. Between 90 and 100 psi
    3. Between 120 and 145 psi
    4. Exactly 60 psi

  7. With the brakes released, what angle should the slack adjuster arm make with the push rod?

    1. Less than 90°
    2. Exactly 180°
    3. Exactly 45°
    4. Greater than 90°

  8. On a type 30 chamber, at how much slack must a brake be adjusted?

    1. 1/4 inch (6.4 mm)
    2. 2 1/2 inches (63.5 mm)
    3. 2 inches (50.8 mm)
    4. 1 inch (25.4 mm)

  9. During a combination pre-trip test you break the supply line. What should happen?

    1. Nothing happens until the foot valve is pressed
    2. Trailer brakes release and the tractor loses all air
    3. The low air warning sounds at 100 psi
    4. Trailer brakes apply and tractor air shuts off at 20 psi or more

  10. What is the most common cause of air brake failure?

    1. Poor brake adjustment
    2. A cracked compressor
    3. Low engine oil
    4. A burnt-out stop lamp

  11. Single unit, engine off, full foot brake held: how much air loss is allowed after the initial drop?

    1. No more than 3 psi in two minutes
    2. No more than 4 psi in one minute
    3. No more than 10 psi in one minute
    4. No more than 1 psi in one minute

  12. How long should it take the compressor to raise pressure from 85 to 100 psi at 600–800 rpm?

    1. Less than ten seconds
    2. Less than two minutes
    3. Less than five minutes
    4. Exactly three minutes

  13. Pulling on the slack adjuster, you measure 1 1/8 in. of push rod travel. What should you do?

    1. Drain the reservoirs and recheck
    2. Have the brake adjusted before driving
    3. Nothing; up to 2 in. is acceptable
    4. Add air pressure and recheck

  14. An automatic slack adjuster is badly out of adjustment. What may bring it back into adjustment?

    1. Up to 12 applications of about 100 psi
    2. One brake application at about 20 psi
    3. Turning the adjusting nut by hand each morning
    4. Driving with the park brake on

Brake adjustment and air brake pre-trip tests: 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. 7 Within an inch of your life (pp. 52–53)

  • Poor brake adjustment is the most common cause of brake failure.
  • A type 30 chamber has 2 1/2 in. of stroke; a correctly adjusted brake has about 1/2 in. of slack, leaving 2 in. in reserve; at 1 in. of slack it must be adjusted.
  • At an 80 psi application, component stretch makes a 1 in. slack brake stroke 1 3/4 in.; a hot drum adds about 1/2 in. more.
  • Lining wear the thickness of three sheets of paper adds about 1/4 in. of stroke and can bottom out the chamber.
  • Poorly adjusted brakes can lengthen stopping distance by up to 75%.
  • Badly adjusted brakes may feel fine under light braking; the problem appears under moderate to heavy braking.
  • Check by prying the slack adjuster to pull the push rod out; 1 in. or more of travel means adjustment is needed.
  • Stroke limits depend on chamber size, e.g. type 30 is 2 in. and type 30 long-stroke is 2 1/2 in.

Slack adjusters and adjustment (pp. 54–57)

  • Slack adjusters link the push rod to the camshaft and keep stroke and lining clearance right, manually or automatically.
  • Automatic slack adjusters must be checked daily: under 1 in. when pulled by hand, under 2 in. when applied.
  • Two to four 100 psi applications a day usually keep automatics adjusted; up to 12 if badly out; still out means a qualified person must repair them.
  • Do not adjust automatics yourself unless a mechanic or trainer who knows that type has trained you.
  • Preferred manual check: mark the push rod released, make a full application, mark again; compare with the 1 1/2 in. maximum.
  • Adjust so there is 1/2 to 3/4 in. of travel when pulled by hand; tap the drum: a ringing sound means clear, a dull sound means drag.
  • Service tests: slack adjusters move freely; released angle more than 90°; at a 20 psi application no less than 90° and equal travel on all.
  • Push rod force stays steady to 2 in. of stroke, then drops sharply.
  • The practical test includes adjusting a manual slack adjuster and explaining how automatic ones are adjusted.
  • At "trucks stop here, check brakes" signs: check compressor pressure, push rod travel, air leaks, gladhands and lines, and drum, bearing and tire temperatures.

Pre-trip procedures (pp. 58–61)

  • Start with the park brake set, wheels blocked and tanks drained if possible; check compressor, belts and lines.
  • Governor: cut-out 120–145 psi; fan the brakes until cut-in, normally 20–25 psi lower but at least 100 psi.
  • Single unit, engine off, full foot application held: no more than 1 psi loss in one minute.
  • Combination unit: no more than 4 psi loss in one minute.
  • Fan the brakes: low air warning at no less than 60 psi; the trailer supply valve should shut off at no less than 20 psi.
  • Build-up test at 600–800 rpm: 85 to 100 psi in less than two minutes (time the primary needle, trailer not charged).
  • Combination: break the service line (no air loss), break the supply line (trailer brakes apply, no loss from the trailer line, tractor air shuts off at no less than 20 psi).
  • Final tests: park brake hold, trailer emergency hold, hand valve, then a foot-brake application while moving slowly.
  • The driver alone is responsible for the braking system being safe before the vehicle moves.

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