Why are spring brakes a reliable parking brake?
Saskatchewan Commercial overview
Saskatchewan Commercial: Dual-circuit systems and spring brakes
Primary and secondary circuits, what happens when one fails, spring-brake chambers, parking-brake valves and the pressures at which spring brakes drag and apply, compounding, blended air, the modulator valve and caging.
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Study reference: SGI Air Brake Manual (2025-26), Ch. 5 Dual air systems — Dual-circuit air system, Spring-brake chambers, Parking-brake system, Two-way check valve, Modulator valve
All 11 questions in this test
Every question and its answer choices. The correct answers and explanations appear when you take the test above.
Why are spring brakes a reliable parking brake?
- They use the highest air pressure in the system
- They can only be released with a wind-off bolt
- They lock the transmission as well as the wheels
- They are held on by spring pressure and need no air
Why should you never take a spring-brake chamber apart?
- It lets moisture into the air system
- The compressed spring can cause serious injury
- It voids the vehicle's inspection decal
- It changes the slack adjuster angle
In a dual-circuit system, the secondary circuit develops a major leak. What happens?
- All service brakes fail at once
- The primary circuit keeps working on its own
- The compressor shuts down to save air
- The spring brakes apply immediately
Which valves separate the primary and secondary reservoirs from the supply reservoir?
- Relay valves
- Quick-release valves
- One-way check valves
- Safety valves
In a dual-circuit system on a two-axle truck, which brakes does the primary reservoir normally operate?
- The spring parking brakes
- The trailer brakes only
- The front service brakes
- The rear service brakes
How are spring brakes applied and released?
- Applied by hydraulic fluid, released by air
- Applied and released by air pressure
- Applied by spring force, released by air pressure
- Applied by air pressure, released by spring force
What does 'compounding' the brakes mean?
- Draining both reservoirs at the same time
- Applying the service brakes while the park brake is on
- Using the hand valve and foot valve together
- Pumping the brakes on a downgrade
With a push-to-release park valve, at about what pressure do spring brakes begin to drag?
- About 10 psi
- About 70 psi
- About 45 psi
- About 100 psi
Why should you release the parking brakes before a full foot-brake application test?
- To avoid compounding the forces on the brake parts
- To keep the low air warning from sounding
- To stop the trailer from rolling forward
- To let the compressor cut in sooner
All air has been lost and the spring brakes have applied. How can the vehicle be moved?
- Pump the foot valve several times until the brakes release
- Repair and recharge, or cage the springs
- Pull the park brake valve out and then push it back in
- Use the trailer hand valve to release them
What is blended air?
- Air taken from both the primary and secondary circuits
- Air that has passed through the air dryer
- Air mixed with oil to lubricate valves
- Air from the trailer and tractor tanks together
Dual-circuit systems and spring 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 ↗
Ch. 5 Dual air systems (pp. 19–23)
- Almost all heavy vehicles use a dual-circuit system so one failure does not cause total brake loss.
- Air goes from the supply reservoir through one-way check valves to the primary and secondary reservoirs.
- Normally the primary reservoir operates the rear brakes and the secondary operates the front brakes.
- The dual foot valve is two valves in one, one section per circuit.
- If one circuit fails, the other keeps working; braking is lost only if both circuits lose air.
- Each circuit has a low pressure warning and a gauge.
Spring brakes and parking (pp. 24–25)
- A spring-brake chamber acts as service brake, emergency brake and parking brake; it is always on the front axle of a tandem and often on both.
- Spring brakes are applied by spring force and released by air pressure, the opposite of a service chamber.
- They make reliable parking brakes because they need no air to hold.
- Never take a spring brake apart; the spring can cause serious injury.
- If air cannot be restored, the spring can be compressed with the wind-off (caging) bolt.
- With a push-to-release park valve, spring brakes may start dragging below about 70 psi and fully apply between 20 and 45 psi.
- If the park button does not pop out, walk back and check: slack adjuster and push rod at 90° means applied.
- Some toggle valves do not apply automatically; when air is restored the brakes release unless the toggle was set to park.
- Compounding: a service application with the park brake on; it can damage components, so release the park brake before a foot application.
- An anti-compound line releases spring-brake pressure equal to the service pressure applied.
Blended air, two-way check valve and modulator (pp. 27–29)
- Blended air for the park control valve comes from both circuits through a two-way check valve.
- The two-way check valve sends air from whichever source has higher pressure, so a single-circuit failure does not apply the spring brakes.
- With a modulator valve, a primary circuit failure lets the foot valve apply the drive-axle spring brakes in proportion to pedal pressure.
- Below 70 psi the spring brakes drag; at about 20 psi the dash valve exhausts the rest and they apply fully.
- After all air is lost, the only ways to move are repairing and recharging, or caging the brakes with the wind-off bolts.
Study the Dual-circuit systems and spring brakes rules as flashcards →
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