Slack Adjusters on Air Brakes 2026: Stroke Checks, Limits

How brake chambers and slack adjusters work

A service brake chamber holds a rubber diaphragm, a pushrod and a return spring. Air pushes the diaphragm, the pushrod moves out (its stroke), and the slack adjuster turns that into rotation at the brake. Each chamber size has a limited stroke, beyond which it produces no braking at all. Automatic slack adjusters take up wear by themselves; manual ones must be readjusted.

How do you check slack adjusters on air brakes?

Chock the wheels, make sure pressure is above 90 psi (pounds per square inch), and release the spring brakes.
Mark the pushrod at the chamber, or measure from a point on the pushrod to a fixed point.
Bring both tank gauges to 90 to 100 psi, shut off the engine and hold the brake pedal fully down.
Measure again. The difference is the applied stroke.
Find the chamber’s size and type, look up its adjustment limit, and treat any brake over the limit as defective.

This is the method in Ontario’s Official Air Brake Handbook, and Ontario’s practical examination asks you to demonstrate it. Ontario’s law requires brakes to be checked for adjustment at least once a day.

Brake chamber adjustment limits

Limits for clamp-type chambers, from Ontario’s handbook. L is a long-stroke chamber; LS is long-stroke with square air ports; DD3 is the bus and coach chamber.

Size	Limit
16	1‑3/4 inch (45 mm)
16 L	2 inch (51 mm)
20	1‑3/4 inch (45 mm)
20 L	2 inch (51 mm)
24	1‑3/4 inch (45 mm)
24 L	2 inch (51 mm)
24 LS	2‑1/2 inch (64 mm)
30	2 inch (51 mm)
30 DD3	2‑1/4 inch (57 mm)
30 LS	2‑1/2 inch (64 mm)
36	2‑1/4 inch (57 mm)

You can find the size from the clamp diameter or the marking on the chamber, though your employer or the owner’s manual is the easier source. Long-stroke chambers have square ports and a trapezoid tag under the clamp bolt.

Who may adjust slack adjusters

Ontario: only certified people may readjust brakes. Drivers can get certified for manual slack adjusters; automatic ones are for certified technicians only.
Manitoba: you need an S air brake endorsement to adjust manual slack adjusters, and only certified technicians may service automatic ones.
New Brunswick: you must hold endorsement E to adjust manual slack adjusters.
Saskatchewan: the practical test includes adjusting a manual slack adjuster and explaining how an automatic one is adjusted.

An automatic slack adjuster that keeps needing readjustment is defective. Ontario’s handbook says to check it often but not to repair it yourself.

Manitoba’s S endorsement →New Brunswick’s endorsement E →

Sources: Ontario Official Air Brake Handbook · Manitoba Public Insurance · New Brunswick Department of Justice and Public Safety and 5 more. Checked October 2026.