Saskatchewan Air Brake common mistakes
Saskatchewan Air Brake · Understand the traps and how to avoid them.
When asked what the low-air warning must do, the candidate says it should come on at 60 psi or says 'about 60' without stating that the warning must activate before the pressure falls below 60 psi and that the compressor governor cut-in is normally around 85 psi.
Candidates memorize the 60 psi number in isolation and never connect it to the governor cut-in/cut-out cycle, so they treat the warning as a single threshold rather than a margin above the cut-in pressure that protects against running out of air between cut-in and cut-out.
What to do instead
Build one pressure map on paper: governor cut-out ~120-125 psi, cut-in ~85 psi, low-air warning at or above 60 psi, spring brakes apply ~40-45 psi. Recite the map until the numbers are tied to their functions, and state the warning threshold as 'must activate before pressure drops below 60 psi.'
The candidate explains the supply circuit, service circuit, and parking/emergency circuit as if they were one continuous line, and cannot say which line charges the trailer tanks versus which line signals a brake application.
The two-line (supply/control) trailer system is taught as a diagram in class, and candidates learn the colors and couplers without tracing where air actually goes, so the red/blue distinction collapses into a memorized label.
What to do instead
Trace air flow out loud from compressor to reservoir to foot valve to relay valve to brake chambers, then separately from reservoir through the supply (emergency/red) gladhand to trailer tanks and from foot valve through the control (service/blue) gladhand to the trailer relay valve. Say which line fails first and what happens when each is disconnected.
During the pre-trip air brake test, the candidate pumps the brake pedal repeatedly to check for leaks, or performs the static leak test with the engine running, and reports a pressure drop that reflects normal compressor cycling rather than a leak.
Candidates confuse the leak-down test with the 'fan the brakes' procedure used to verify the low-air warning and spring brake application, and they leave the engine idling because that is how they normally drive.
What to do instead
Sequence the tests in a fixed order and say the engine state for each: with the system fully charged and the engine off, hold the brake pedal and watch for a drop beyond the allowed limit over one minute (single vehicle) and check the trailer the same way; separately, with the engine off, fan the brakes down to verify the warning and spring brake set. Practise the order until it is automatic.
The candidate does not perform or cannot describe the tug test, or performs it with the trailer brakes released, and then reports the coupling as secure.
The tug test looks trivial, so candidates skip rehearsal and only read about it; without the trailer spring brakes set, the tractor can pull against nothing and the test proves nothing.
What to do instead
Rehearse the exact steps: back under the trailer, hear and feel the fifth wheel lock, set the trailer parking brakes, put the tractor in a low gear and pull gently against the locked trailer, then confirm the coupling by visual check of the jaws and the release handle. State that the trailer brakes must be set for the tug to be meaningful.
The candidate adjusts the slack adjusters by feel or assumes automatic slack adjusters never need attention, and cannot explain the pushrod stroke limit or the applied-stroke measurement method.
Automatic slack adjusters are marketed as maintenance-free, and candidates extrapolate that to 'never check,' while manual adjustment is learned as a physical skill that is hard to practise without a vehicle.
What to do instead
Learn the measurement procedure: with the system charged and the brakes fully applied, measure pushrod travel at the chamber, compare against the limit for the chamber type and size, and know that a stroke over the limit means the brake is out of adjustment regardless of adjuster type. Explain that automatic adjusters only adjust during a normal apply-and-release cycle and will not correct a badly out-of-adjustment brake.
When asked what to do if the low-air warning sounds while driving, the candidate says to keep driving to the nearest shop or to downshift and use the engine brake to save air.
Candidates treat the warning as an advisory light rather than a mandatory stop signal, and they have not connected the warning threshold to the spring brake application point that follows shortly after.
What to do instead
State the response as a fixed script: stop as soon as it is safe, do not continue to a destination, and understand that once pressure falls to the spring brake application range the brakes will set and cannot be released until pressure is restored. Explain that engine braking and downshifting help control speed but do not restore air pressure.
The candidate describes the spring brakes as the service brakes and cannot explain that they are held off by air pressure and apply when pressure is lost, or confuses the parking brake with the emergency brake function.
Spring brakes are invisible in normal driving, so candidates learn them as a parking feature and never internalize the fail-safe inversion that makes loss of air a braking event rather than a loss of braking.
What to do instead
Explain the mechanism in one sentence: a large spring pushes the brake on, and air pressure compresses the spring to release it, so any loss of air applies the brakes. Then distinguish the three functions: service (foot valve modulates air), parking (push-pull valve exhausts air), and emergency (automatic application on pressure loss).
The candidate cannot explain what the relay valve does or where it is located, and describes the foot valve as sending air directly to the brake chambers.
The relay valve is buried in the plumbing and its role is abstract, so candidates memorize component names from a list without learning the signal-versus-supply distinction.
What to do instead
Describe the relay valve as a remote-controlled valve: the foot valve sends a small control signal, and the relay valve admits a large volume of air from a nearby reservoir to the chambers, shortening lag time and reducing the volume that must travel the length of the vehicle. State that it is mounted near the axles it serves.
The candidate drains air tanks only by opening the petcock briefly at the start of the day and does not know which tank is the wet tank or why it is drained first, or reports that automatic drain valves make manual draining unnecessary.
Draining is treated as a ritual rather than a diagnostic, so candidates never learn that the wet tank collects the most moisture and that what comes out of each tank tells you about the compressor and air dryer.
What to do instead
Learn the tank order and purpose: compressor feeds the wet (supply) tank first, which collects condensate, then the dry tanks feed the circuits. Drain the wet tank first and inspect what comes out; explain that automatic drains can fail and manual checks remain part of the pre-trip. Note that excessive water or oil points to air dryer or compressor problems.
The candidate answers the brake adjustment and leak-rate questions with a single number remembered from a course handout, without stating the units, the time interval, or whether the limit is for a single vehicle or a combination.
Leak limits are learned as isolated figures, and candidates do not rehearse stating the full condition (pressure drop, time, system type), so the answer sounds right but cannot be defended.
What to do instead
Practise answering in a complete sentence that names the condition: state the allowed pressure drop, the hold time, whether the engine is off, and whether the reading applies to the tractor alone or the combination. If you are unsure of a specific limit, say what you would check in the Saskatchewan commercial vehicle manual rather than guessing a number.