Using the applied stroke method on a Type 30 chamber, when does the brake need adjusting?

When applied stroke exceeds 1¾ inches

When applied stroke is under ½ inch

When applied stroke is 1¼ inches

BC Class 1–4: Air brake adjustment

Pushrod stroke limits, checking manual slack adjusters by the applied and pry methods, adjusting them, automatic slack adjusters and common misconceptions.

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14 questions
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Air brake adjustment · Question 1Score 0/0

Does a 90-degree slack adjuster angle when applied prove the brake is adjusted?

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Study reference: Driving Commercial Vehicles (ICBC, 2024 edition), Ch. 9 Air brake adjustment — Checking s-cam brakes, Adjusting a manual slack adjuster, Automatic slack adjusters

All 14 questions in this test

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

Does a 90-degree slack adjuster angle when applied prove the brake is adjusted?
1. Yes, on automatic adjusters
2. No; it depends on pushrod length
3. Yes, a 90-degree angle always proves adjustment
4. Only on trailer axles
After tightening a manual slack adjuster until the linings touch the drum, how far do you back off?
1. About 1/3 of a turn
2. About 1/8 of a turn
3. Two full turns
4. Not at all
What is the correct running clearance between lining and drum?
1. No clearance at all
2. About one inch
3. A few thousandths of an inch
4. About half an inch
Since when have all air-braked commercial trucks and trailers been built with automatic slack adjusters?
1. 1980
2. 2000
3. 1996
4. 2010
Using the applied stroke method on a Type 30 chamber, when does the brake need adjusting?
1. When applied stroke is 1 inch
2. When applied stroke is under ½ inch
3. When applied stroke is 1¼ inches
4. When applied stroke exceeds 1¾ inches
While adjusting a manual slack adjuster, the pushrod starts coming out of the chamber. What does that mean?
1. You are turning the bolt the wrong way
2. The brake is correctly adjusted
3. The chamber diaphragm has failed
4. You should keep turning harder
What is the maximum stroke of a standard Type 30 brake chamber?
1. 2½ inches (65 mm)
2. 1¾ inches (45 mm)
3. 1 inch (25 mm)
4. 4 inches (100 mm)
What does a visible red marking on a new pushrod mean?
1. The brake was adjusted correctly this morning
2. The brake is badly out of adjustment
3. The pushrod is the wrong size
4. The chamber is a long-stroke type
Using the pry method, what free stroke is acceptable?
1. 1 to 1½ inches
2. Less than 1/8 inch
3. ½ to ¾ inch
4. 1¾ to 2 inches
Who is ultimately responsible for brake adjustment before the vehicle moves?
1. The maintenance crew
2. The driver
3. The vehicle owner only
4. The CVIP inspector
Why can a poorly adjusted brake fail on a long downgrade even if it seemed fine when cool?
1. The linings swell and lock
2. The slack adjuster tightens itself
3. Hot air raises system pressure past the safety valve
4. Hot drums expand and the chamber bottoms out
May you retrofit manual slack adjusters on a vehicle built with automatic ones in B.C.?
1. Yes, on trailers only
2. No, it is illegal
3. Yes, with a mechanic's note
4. Yes, if they are new
How often must you check manual and automatic slack adjusters?
1. Only when the brakes feel weak
2. Weekly, at the yard
3. Only at the annual inspection
4. Daily, during the pre-trip inspection
An automatic slack adjuster shows excessive stroke. What should happen?
1. Remove it and fit a manual one
2. Have a qualified mechanic repair it
3. Ignore it; it will reset itself
4. Adjust it by hand and drive on

Air brake adjustment: 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 ↗

It's critical, it's the law (p. 189)

The foundation brakes are the most important part of any braking system; even a well-designed air system cannot stop a vehicle whose brakes are out of adjustment.
Brake failures and runaways are almost never caused by the air system itself; missing maintenance or drivers not checking brakes daily is far more common.
Daily pre- and post-trip inspections, including checking and adjusting brakes, are mandatory, and the driver may be held responsible for badly adjusted brakes.
Federal and provincial law require you to check manual and automatic slack adjusters daily during the pre-trip inspection.
You must also check your brakes before descending steep grades posted with regulatory signs.
A maintenance crew may exist, but the driver is ultimately responsible for the brakes before the vehicle moves.
Operating with defective brakes, including excessive pushrod stroke, can bring a fine of up to $2,000.

S-cam brake released, applied and out of adjustment (p. 189–191)

The Type 30 chamber is the most common on trucks and trailers; it has a maximum stroke of 2½ inches (65 mm); a long-stroke Type 30LS has three inches.
A correctly adjusted brake strokes less than half its maximum when applied.
Slack adjustment means keeping chamber pushrod travel within tolerance so the brakes work.
Pushrod travel is the distance the pushrod comes out of the chamber when the brakes apply.
Correct running clearance between lining and drum is only a few thousandths of an inch, about three sheets of paper.
Check pushrod travel at least daily; brakes on the same axle should have similar travel left and right.
A brake with excessive travel may still seem to work with a cool drum and normal pressure, giving false security.
Cast iron drums expand when hot, so the chamber strokes farther; a badly adjusted brake can bottom out and lose most of its power.
On long downgrades, hot drum expansion with poorly adjusted brakes can cause total loss of braking and a runaway.

Checking s-cam brakes with manual slack adjusters (p. 191–193)

Park safely with wheels blocked, air at full pressure, spring parking brakes released, engine off so you can hear leaks.
Applied stroke method (used by roadside inspectors): mark the pushrod at the chamber (or measure clevis pin to chamber face) with brakes released and reservoir at least 100 p.s.i.
Then hold a full application of 90 to 100 p.s.i. (620–690 kPa) and measure again; usually two people are needed unless you have a device to hold the brakes on.
A good applied stroke is 1 to 1½ inches (25–38 mm); more than 1¾ inches (45 mm) means the brake needs adjusting.
Pry method: with system air over 100 p.s.i. and parking brakes released, push the pushrod fully in, mark it, pry it out with a bar and measure.
A good free stroke is ½ to ¾ inch (12–20 mm); more than ¾ inch (20 mm) means adjustment is needed.
These limits are maximums for a Type 30 clamp chamber; adjust when measurements approach them.
New pushrods have a marking, usually red, that shows when travel is excessive; if it is visible the brake is dangerously out of adjustment; do not wait for it to appear.
Tools: flashlight, chalk or marker and ruler, a pry tool, and a wrench for manual slack adjusters; wear sturdy clothing, hard hat and eye protection.
Also look for s-cam bushing wear, s-cam spline wear, slack adjuster play (worn gear) and clevis pin wear.

Adjusting a manual slack adjuster (p. 194–195)

Most manual slack adjusters have a spring-loaded locking sleeve that must be pressed in and held while turning the adjusting bolt, usually with a 9/16-inch wrench.
Turning the right way makes the camshaft rotate in the same direction as when the brakes apply.
If turning the bolt pulls the pushrod out of the chamber, you are turning the wrong way; stop.
The pushrod and slack arm should not move in or out while you turn the bolt; if the pushrod goes into the chamber, the previous adjustment was done wrong.
Turn until solid resistance (linings touching the drum), then back off about 1/3 of a turn and make sure the locking sleeve re-engages.
The correct direction may be clockwise or counter-clockwise; on tandems one axle often adjusts opposite to the other.
After adjusting, re-measure free stroke to confirm clearance.

Automatic slack adjusters (p. 195–197)

All air-braked commercial trucks and trailers have been built with automatic slack adjusters since 1996.
They adjust themselves during normal braking but must still be checked at every pre-trip inspection.
Excessive stroke with an automatic slack adjuster usually means it failed, was installed wrong, or there is a foundation brake problem; it needs a qualified mechanic.
Manually adjusting an automatic slack adjuster is dangerous: it gives false security, the adjuster soon resets, and repeated adjustment wears it out and can mask other problems.
Manufacturers generally recommend a mechanic check them at every chassis lube, every 40,000 km or every three months, whichever comes first.
Check stroke with a 90 to 100 p.s.i. application; without an application gauge, shut the engine off and pump reservoir pressure down to 90–100 p.s.i., then make a full application.
Automatic adjuster strokes are usually slightly longer; on a Type 30 the brake needs repair if free stroke exceeds ¾ inch (20 mm) or applied stroke exceeds two inches (50 mm).
Emergency adjustment is a last resort only (for example, to reach a service depot); record it on the post-trip report.
Hexagonal bolt type: turn clockwise until the lining contacts the drum, then back off ½ turn; ratcheting is normal.
Square bolt type: disengage the spring-loaded pawl first (pry up the button about 1/32 inch or remove cap, spring and pawl), turn counter-clockwise to contact, then ½ turn clockwise.

Air brake adjustment misconceptions (p. 197–198)

False: an 8 to 12 p.s.i. pressure drop on a full application shows adjustment; large reservoirs and coarse gauges make this useless.
False: a 90-degree slack adjuster angle when applied proves good adjustment; the angle depends more on pushrod length and may vary plus or minus 10 degrees by design.
False: clockwise always sets up the brakes; it depends on the adjuster's orientation.
False: open parking and trailer supply valves prove spring brakes are off; some stay open with as little as 20 p.s.i., so have 90 to 100 p.s.i. and all parking brakes released before checking.
False: manual slack adjusters are better; a Transport Canada study found vehicles with manual adjusters were put out of service 150 per cent more often.
It is illegal in B.C. to retrofit manual slack adjusters on a vehicle built with automatic ones.
There is no reliable substitute for physically checking brake adjustment.

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