How do you avoid compounding during a pre-trip brake test?
BC Class 1–4 overview
BC Class 1–4: Spring parking brakes
Why parking needs mechanical force, how spring brakes apply and release, compounding, caging, blended air and safety actuators.
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- 11 questions
- Topic drill
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Study reference: Driving Commercial Vehicles (ICBC, 2024 edition), Ch. 8 Air brakes — Parking brakes, Spring parking brakes, Compounding, Blended air
All 11 questions in this test
Every question and its answer choices. The correct answers and explanations appear when you take the test above.
How do you avoid compounding during a pre-trip brake test?
- Apply the spring brakes first, then press the pedal hard
- Pump the foot valve quickly
- Release the spring brakes before a full application
- Use the trailer hand valve only
What must you do before caging a spring brake?
- Release the service brakes
- Drain the wet tank
- Block the wheels
- Start the engine
Which parking brake cannot be compounded?
- An s-cam drum brake
- A safety actuator
- A wedge brake
- A spring parking brake
About how much force does the coil spring in most spring brake chambers exert?
- 5,000 to 6,000 pounds
- 1,500 to 2,000 pounds
- About 500 pounds
- 150 to 200 pounds
What colour is the dash parking brake control on most vehicles?
- Green
- Red
- Blue
- Yellow
What is blended air in a dual system?
- Air from the compressor and the wet tank
- Parking control fed by the higher reservoir
- Air shared between tractor and trailer
- Air mixed with alcohol vapour to stop freezing
How are spring brakes applied and released?
- Applied by hydraulic pressure
- Applied by air, released by spring
- Applied by spring force, released by air
- Applied and released by air
On which axles are spring parking brakes mounted?
- Trailer axles only
- All axles
- Steering axles only
- Rear axles only
What is compounding?
- Using the hand valve and the foot valve at the same time
- Draining two reservoirs at once
- Heavy service braking on top of applied spring brakes
- Connecting two trailers to one tractor
Why is it illegal to park a vehicle using air pressure alone?
- A leak could release the brakes
- It overheats the brake drums
- It wears out the compressor
- It drains the battery
At what system pressure may spring brakes apply fully on their own?
- Between 60 and 80 p.s.i.
- Only at zero p.s.i.
- Between 90 and 100 p.s.i.
- Between 20 and 45 p.s.i.
Spring parking brakes: 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 ↗
Parking brakes (p. 160–161)
- Parking with air pressure alone is unreliable and illegal; any leak or failed hose, diaphragm or valve could let the vehicle roll away.
- Regulations require parking force to be held mechanically and be unaffected by loss of air pressure.
- The most common air-system parking brake is the spring parking brake; the second type, the safety actuator, is found mostly on some highway coaches and intercity buses.
Spring parking brakes (p. 161–163)
- A spring parking brake is an extra chamber piggy-backed on the rear of a service brake chamber, containing a powerful coil spring, a second diaphragm and an intermediate pushrod.
- Spring parking brakes are mounted on rear axles only, never on steering axles.
- The coil spring in most spring brake chambers exerts 1,500 to 2,000 pounds.
- Service brakes are applied by air; spring brakes are applied by spring force and held off (released) by air pressure.
- Spring brakes act through the same pushrod, slack adjuster and foundation brake as the service brake.
- If system pressure falls below about 60 p.s.i. the spring brakes may start to drag, and at 20 to 45 p.s.i. (138 to 310 kPa) they may apply fully on their own.
- In some systems spring brakes apply gradually as pressure drops and may start applying at 90 p.s.i. or higher.
- Their effectiveness for stopping or holding depends on the brakes being kept in adjustment.
- The dash parking brake control is usually a yellow button: push in to release the spring brakes, pull out to apply them; some vehicles use a flip switch.
- Only qualified people should service spring brake chambers; keep away from a chamber with dents or cracks because the spring could break out.
Compounding (p. 164)
- With spring brakes applied, up to about 2,000 lb is on the brake parts; a heavy service application adds up to about 3,000 lb more, for about 5,000 lb.
- This adding of forces is compounding; it can damage slack adjusters, s-cams, chamber mounting bolts, shoe rollers, shoes and drums.
- Release the spring parking brakes before making a heavy service application, such as during a pre-trip test.
- Light applications under 30 to 40 p.s.i. to keep the vehicle from rolling while spring brakes are set or released are not harmful.
Manual release (caging) (p. 164–165)
- Caging means mechanically compressing the spring with a release bolt so the spring brake is off; it is meant for mechanics doing repairs or, in an emergency, to tow a vehicle that has lost all air.
- Always block the wheels before caging; once caged there is no parking force at that wheel.
- A release bolt is inserted, given a quarter turn to lock, and the nut turned to compress the spring; instructions are usually on the housing.
- After manually releasing spring brakes, call a tow truck to move the vehicle.
Spring brakes in dual systems: blended air (p. 165–167)
- A two-way check valve feeds the parking brake control from whichever service reservoir has the higher pressure; this is called blended air.
- The two-way check valve has two inlet ports, one delivery port and a free-floating shuttle that moves away from the higher-pressure inlet.
- Blended air keeps the spring brakes from applying automatically if either reservoir loses all its air.
- One way to test the two-way check valve: drain the primary reservoir and confirm secondary pressure does not bleed back into it.
- An optional spring brake modulator senses primary loss and, when you press the foot valve, releases spring brake air in proportion to help the front brakes stop the vehicle.
- All vehicles must meet federal emergency stopping standards even with a partly failed system; after stopping, park with the parking brake control valve.
Safety actuator parking brakes (p. 167–168)
- Safety actuators, used on many buses and coaches, use a one-way locking mechanism instead of a coil spring, with one diaphragm for service and one for parking, and a separate parking reservoir.
- Pulling the dash control applies air to the parking diaphragm and engages the lock, which holds the pushrod out even if all air is lost.
- If more than 4 p.s.i. (27.6 kPa) has been lost from the parking reservoir, the brakes will not release until a heavy service application frees the lock.
- Safety actuators cannot be compounded, and they apply automatically only if the parking reservoir loses pressure, not the service reservoirs.
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.
- 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.