What must every trailer's emergency system do if the trailer separates?
BC Class 1–4 overview
BC Class 1–4: Tractor-trailer air systems
Supply and control lines, glad hands, trailer emergency application, the tractor protection system, the trailer hand valve and bobtail tractors.
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Study reference: Driving Commercial Vehicles (ICBC, 2024 edition), Ch. 8 Air brakes — Tractor-trailer air brake systems, Tractor protection, Hand valve, Bobtail tractors
All 12 questions in this test
Every question and its answer choices. The correct answers and explanations appear when you take the test above.
What must every trailer's emergency system do if the trailer separates?
- Release all trailer brakes
- Fully apply the trailer brakes
- Lower the landing gear
- Sound a warning horn
Which line fills the trailer reservoirs at full tractor pressure?
- The supply (emergency) line
- The electrical cable
- The control (service) line
- The hydraulic line
What is the trailer hand valve meant for?
- Emergency stops at highway speed
- Applying only the trailer brakes, such as when coupling
- Normal braking in traffic
- Parking the trailer overnight on a slight grade
How long must dynamited brakes on a trailer without spring brakes stay applied?
- At least 15 minutes
- At least one hour
- At least 2 minutes
- Until the driver releases them
How should you park a tractor-trailer?
- Apply the trailer hand valve, then leave the engine running
- Leave it in gear with the engine idling
- Tractor brakes set, trailer supply closed, wheels blocked
- Pump the foot valve and set the hand valve
Within what tractor pressure range must trailer air shut off automatically?
- 20 to 45 p.s.i.
- 90 to 100 p.s.i.
- Below 5 p.s.i.
- 60 to 90 p.s.i.
The trailer spring brakes will not release and the service brakes do not work. What is a likely cause?
- The landing gear is down
- The supply and control lines are reversed
- The trailer is overloaded on its rear axle
- The ABS lamp has failed
What shape and colour is most trailer air supply valves?
- A blue circle
- A yellow diamond
- A green square
- A red octagon
What is meant by "dynamiting" the trailer brakes?
- Pumping the trailer brakes quickly on a slippery road
- Full brake application when supply air is lost
- Overheating the brakes on a hill
- Releasing the brakes with the hand valve
Why can a bobtail tractor's rear brakes lock up easily?
- The governor cuts out early
- There is little weight on the drive axles
- The trailer air supply valve stays open
- The steering axle has no brakes
Which colour usually marks the control (service) glad hand?
- Blue
- Yellow
- Red
- Green
What does the tractor protection system do in a trailer breakaway?
- Shuts off air loss so the tractor can stop
- Opens the safety valve
- Applies the steering axle brakes only
- Releases all tractor brakes
Tractor-trailer air systems: 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 ↗
Tractor-trailer air brake systems (p. 168–169)
- A trailer air system has foundation brakes, chambers, reservoirs and valves but no compressor; it relies on the tractor for air and for apply/release commands.
- The supply line (also called the emergency line) fills the trailer reservoirs at full tractor reservoir pressure.
- The control line (also called the service line) carries the brake signal from the tractor.
- Glad hands are the quick couplers on the air lines; blue usually marks the control line and red the supply line.
- When not in use, fasten glad hands to dummy couplers or to each other to keep out dirt and stop chafing or bouncing.
- Every trailer must have an emergency system that fully applies its brakes if it separates from the tractor.
Trailers with a relay emergency valve (p. 169–170)
- Trailers without spring brakes use a relay emergency valve, which charges the trailer reservoir and relays the control signal at about the same pressure.
- If supply line pressure is lost, the relay emergency valve applies full trailer reservoir pressure to the service chambers: "dynamiting" the trailer brakes.
- Dynamiting also happens whenever the glad hands are disconnected or the driver closes the trailer air supply valve.
- Safety standards require these dynamited brakes to stay applied at least 15 minutes; they hold only while the trailer reservoir keeps pressure.
- Small leaks in trailers without spring brakes can let a parked trailer roll away, so always block the wheels.
- Highway trailers use one reservoir and one relay emergency valve for a single or tandem axle; some tandem logging trailers have one per axle.
Trailers with spring parking brakes (p. 170–172)
- A trailer spring brake valve fills the reservoir(s) and controls the spring brakes; a relay valve handles the service brakes.
- Some systems fill the reservoir before releasing the spring brakes; others release the spring brakes first.
- On loss of supply line pressure, the spring brake valve exhausts the spring chambers so the springs apply (dynamiting).
- Signs the supply and control lines are reversed: the trailer spring brakes will not release with the dash control and the trailer service brakes do not work.
- Always do a tug test after coupling, and check with a flashlight that the fifth wheel jaws are locked first.
Tractor protection (p. 172–173)
- If the trailer breaks away, air rushes out of the broken supply line (and the control line when braking).
- The tractor protection system is the dash trailer air supply valve plus a tractor protection valve usually behind the cab; all air to the trailer passes through it.
- It shuts off air loss from the tractor in a breakaway so the driver keeps enough pressure to stop.
- Some systems shut off immediately; some let tractor pressure drop to as low as 20 p.s.i. (138 kPa) before closing.
- Safety standards require automatic shutoff of trailer air when tractor pressure falls to between 20 and 45 p.s.i. (138 and 310 kPa); with blended air this happens when the higher service reservoir reaches that range.
- A peace officer may put the vehicle out of service if tractor pressure can drop below 20 p.s.i. without the trailer air supply valve closing.
- A peace officer may put a trailer out of service if its brakes do not apply when the trailer air supply valve is closed.
- Check the tractor protection system in every daily pre-trip inspection.
- Most trailer air supply valves are an octagon-shaped red button on the dash.
- With no trailer attached, the trailer air supply valve stays closed so no air is lost through the open glad hands.
- If only the control line separates, nothing happens until the brakes are applied; then the protection system closes to save tractor air.
Hand valve and two-way check valves (p. 173–175)
- A foot valve application sends about the same pressure to tractor and trailer brakes (for example 20 p.s.i. to both).
- The hand valve (on the steering column or dash) applies the trailer brakes alone, for example when coupling; it is spring-loaded and returns to off when released.
- Do not use the hand valve for normal or emergency braking; use the foot valve.
- Never use the hand valve for parking: set the tractor parking brakes, close the trailer supply valve and block the trailer wheels.
- Two-way check valves send the higher of the hand valve or foot valve pressure to the trailer control line.
- A tractor equipped to tow adds a trailer air supply valve, tractor protection valve, hand control valve and a pair of two-way check valves.
- In most dual systems the yellow parking brake button is interlocked with the red trailer supply button, so pulling yellow applies all tractor and trailer parking brakes.
- Some tractors have a third, round blue button that controls the tractor parking brakes on their own.
- With a tractor primary failure, the secondary still applies the tractor front brakes and signals the trailer brakes through the two-way check valves.
Bobtail tractors (p. 174)
- Driving a tractor without a trailer is bobtailing.
- With little weight on the drive axles, a bobtail tractor's rear brakes lock up easily, even with a light application.
- A bobtail proportioning system reduces drive axle brake pressure by as much as 75 per cent when bobtailing, while the steering axle gets full pressure.
- A bobtail tractor with this system stops shorter with better control, but needs more pedal pressure than normal.
Other topics
- Licence classes, endorsements and testingClass 1 to 4 and what each covers, minimum ages, the air brake (code 15) and heavy trailer (code 20) endorsements, the driving record and medical rules, learner supervision, MELT and retest waiting periods.
- Heavy vehicle brakingHow brakes turn motion into heat, stopping time and following distance, speed and weight effects, brake fade, retarders, downgrades, water, runaway lanes and ABS.
- Basic commercial driving skillsFollowing distance, danger zones, off track and turns, curves, roundabouts, backing, mirrors, emergency vehicles, double-clutching, railway crossings and night driving.
- Tires, warnings and personal safetyTire pressure and tread limits, wheel checks, required braking systems, flashing lights, warning devices for disabled vehicles, carbon monoxide, seatbelts, phones, impoundment and fire fighting.
- Fuel-efficient drivingStarting and warming up a diesel, smart shifting and speed, idling, tire pressure, en route checks and vehicle choices that save fuel.
- Backing and handling trailersBacking a semi-trailer, rollovers and load height, double trailers, trailer whipping, parking a trailer and weight distribution on a tractor-trailer.
- Coupling and uncouplingCoupling and uncoupling a fifth wheel step by step, the tug test, landing gear, pintle hitches, safety chains and drawbars.
- Loading and weight distributionWhere to place heavy loads on a truck, axle load shares for freight, dump and mixer trucks, covered aggregate loads, livestock and liquid tank loads.
- Cargo securementThe North American cargo securement standard, tiedown condition, how many tiedowns and how strong, binders, front-end structures, load projections and vehicles carried as cargo.
- Dimensions, weights and permitsHeight and width limits, mirrors, GVW and licensed GVW, overload and oversize permits, pilot cars and reporting to weigh scales.
- Dangerous goods and signs for commercial driversDangerous goods training and documents, reporting spills, placard classes, CANUTEC, runaway lanes, low clearance, transit priority and zone signs.
- Operating a busBus passenger safety, leaving the curb, transit right-of-way, highway stops, fares, standing passengers, required equipment, baggage and the bus pre-trip.
- School busesWhich licences cover school buses, daily brake checks, school bus equipment and signs, loading and unloading, flashing red lights and railway crossings.
- Taxis, limousines and ride-hailingClass 4 restricted, taxi and limousine definitions, passenger transportation licences, seating, duty to carry and when you may refuse, jump seats and passengers with disabilities.
- Ambulances and special vehiclesLicences for ambulances and special vehicles, when emergency vehicles may use lights and sirens, yielding to them, and equipment rules that depend on seating capacity.
- Hours of serviceOn-duty and off-duty time, daily and shift limits, cycles and resets, deferral, sleeper berths, personal use, ELDs and records of duty status, the 160 km exemption and other jurisdictions.
- Air brake componentsWhy air brakes, brake chambers and slack adjusters, s-cam brakes, compressor, governor, reservoirs, foot valve, air dryers and ABS.
- Dual air brake systemsThe dual foot valve, primary and secondary circuits, one-way check valves, the low-air warning, quick release and relay valves, and what happens when one circuit fails.
- Spring parking brakesWhy parking needs mechanical force, how spring brakes apply and release, compounding, caging, blended air and safety actuators.
- Air brake adjustmentPushrod stroke limits, checking manual slack adjusters by the applied and pry methods, adjusting them, automatic slack adjusters and common misconceptions.
- Trip inspections and reportsWhen inspections are required, which vehicles need written reports, cargo securement re-checks, report rules, pre-trip procedure, brake checks before hills and en route checks.
- Class 1 pre-trip inspectionThe Class 1 road test pre-trip: timing, slack adjuster set-up, fifth wheel and landing gear checks, leakage limits with trailers, tug tests and breakaway checks.
- Air brake pre-trip testThe air brake pre-trip on the road test: draining reservoirs, governor cut-out and cut-in, low-air warning, trailer supply valve closing, build-up time and leakage limits.