The low-air warning comes on while you drive. What should you do?
BC Class 1–4 overview
BC Class 1–4: Dual air brake systems
The 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.
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- 11 questions
- Topic drill
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Study reference: Driving Commercial Vehicles (ICBC, 2024 edition), Ch. 8 Air brakes — Dual air brake systems, Low-air warning, Quick release and relay valves
All 11 questions in this test
Every question and its answer choices. The correct answers and explanations appear when you take the test above.
The low-air warning comes on while you drive. What should you do?
- Release the spring brakes
- Keep going to the next town
- Pump the brakes to raise pressure
- Stop and find the cause before going on
The foot valve sends a 20 p.s.i. signal to a relay valve. What pressure does the relay valve deliver to the chambers?
- About 100 p.s.i.
- About 40 p.s.i.
- About 20 p.s.i.
- About 10 p.s.i.
What do the one-way check valves between the reservoirs do?
- Stop air flowing back out of the service tanks
- Warn the driver of low air
- Exhaust air quickly on release
- Raise the pressure in the wet tank when braking
What does a dual foot valve do?
- Controls the spring brakes
- Doubles the braking force at every wheel
- Splits the system into two sections
- Supplies the trailer only
What is the purpose of a quick release valve?
- To fill the trailer reservoirs
- To apply the brakes faster in an emergency
- To lock the brakes when parked
- To exhaust air close to the chambers
What does a relay valve reduce?
- Wear on the brake linings
- Fuel used by the compressor
- Air pressure in the wet tank
- Lag on both application and release
In many dual systems, which brakes does the primary section work?
- The drive (rear) axle brakes
- The spring parking brakes
- The steering axle brakes
- The trailer brakes only
The low-air warning must come on before pressure drops below what?
- 40 p.s.i.
- 60 p.s.i.
- 90 p.s.i.
- 20 p.s.i.
A line rupture empties the primary reservoir. What protects the secondary reservoir?
- The safety valve
- The one-way check valve
- The quick release valve
- The governor
How many reservoirs do most dual air brake systems have?
- Three
- Five
- Two
- One
The primary section of a dual system fails. Which brakes still apply?
- Only the drive axle brakes
- Only the steering axle brakes
- All brakes normally
- No brakes at all
Dual air brake 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 ↗
Dual air brake systems (p. 155–156)
- Dual air brake systems have been used since the mid-1970s; cars have had dual brake systems since 1968.
- The dual foot valve is two control valves worked by one pedal, splitting the system into two independent sections with their own supply, delivery and exhaust ports.
- The primary section is nearest the pedal and in many systems works the drive (rear) axle brakes; the secondary usually works the steering axle brakes.
- Most dual systems have three reservoirs: the supply reservoir (wet tank) and two service reservoirs called primary and secondary.
- If either section fails completely, the driver can still make a controlled stop with the foot valve, with less maximum braking.
- Some dual systems connect the low-air warning to the supply reservoir; others connect one to each service reservoir.
Components of a dual system (p. 156–157)
- Most contaminants settle in the supply reservoir, so the primary and secondary reservoirs give cleaner air to every component except the governor.
- One-way check valves let air flow from the supply reservoir into the primary and secondary reservoirs but not back.
- The check valves keep primary and secondary air from being lost if the compressor, its discharge line or the supply reservoir fails.
- Every air-brake vehicle has at least one dash gauge showing service reservoir pressure; many use one gauge with two needles (primary and secondary).
- Many vehicles also have an application pressure gauge showing how much air the foot valve is applying.
Low-air warning device (p. 157–158)
- Every air-brake vehicle must have a low-air warning that comes on before pressure drops below 60 p.s.i. (414 kPa); many come on above 60.
- Low pressure can come from a leak or from braking repeatedly faster than the compressor can refill.
- Most are a dash light, often with a buzzer; older vehicles may have a wig-wag that drops into view near the top of the windshield.
- Some wig-wags retract automatically above 60 p.s.i.; others must be pushed back up by hand.
- When the warning comes on, stop and find the cause; the roughly 60 p.s.i. left is enough to stop if you act promptly; do not continue until the system is safe.
Quick release and relay valves (p. 158–160)
- A quick release valve lets brakes release quickly by exhausting chamber air near the chambers rather than back at the foot valve.
- On release, the bigger air burst comes from the quick release valve; only the line between it and the foot valve vents at the foot valve.
- Quick release valves can be on front brakes, rear brakes, spring parking brakes or anywhere fast exhaust is needed.
- A relay valve is a remote-controlled valve, mounted on the frame near the chambers it serves, that cuts lag on both application and release.
- A relay valve gets reservoir air through a large supply line and delivers it at the same pressure as the foot valve's control signal (for example 20 p.s.i. in, about 20 p.s.i. out).
- On release only the control line air vents at the foot valve; chamber air exhausts at the relay valve.
- Relay valves serve two or four chambers and are mainly on rear axles.
Dual system with a failed section (p. 160)
- If a line rupture empties the primary reservoir, the one-way check valve protects the secondary reservoir.
- With a primary failure, only the steering axle brakes apply, so stopping takes longer.
- With a secondary failure, the rear axle brakes still work but the steering axle brakes do not.
- The compressor keeps pumping but its output escapes at the rupture.
- If the low-air warning comes on, stop immediately and do not go on until the system is repaired.
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.
- Spring parking brakesWhy parking needs mechanical force, how spring brakes apply and release, compounding, caging, blended air and safety actuators.
- Tractor-trailer air systemsSupply and control lines, glad hands, trailer emergency application, the tractor protection system, the trailer hand valve and bobtail tractors.
- 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.