When does the service line contain air?
Saskatchewan Commercial overview
Saskatchewan Commercial: Tractor and trailer air systems
The tractor protection system, trailer-supply valve, supply and service lines, gladhands, the hand valve, relay emergency valve, breakaways and line ruptures, bobtail valve and trailer spring brakes.
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Study reference: SGI Air Brake Manual (2025-26), Ch. 6 Tractor system/trailer towing system
All 12 questions in this test
Every question and its answer choices. The correct answers and explanations appear when you take the test above.
When does the service line contain air?
- Whenever the trailer-supply valve is open
- Only when a brake application is made
- Whenever the engine is running
- Only when the parking brakes are applied
Which function belongs to the relay emergency valve on a trailer?
- It dries the air before it reaches the trailer tank
- It controls the tractor's spring parking brakes
- Applies the trailer brakes if the supply line parts
- It lowers pressure to the trailer's front axle
Your tractor-trailer starts to skid. When might gentle use of the hand valve help?
- Whenever the road is icy
- When the tractor skids, to help straighten the unit
- Only to hold the unit still when parked on a grade
- When the trailer is skidding sideways behind you
The supply line to the trailer ruptures. What happens to the trailer brakes?
- They go into an emergency application at once
- They release completely
- They wait until the next foot application
- They work only with the hand valve
What is 'dynamiting'?
- An emergency trailer brake application from sudden air loss
- A slow, steady leak from the service line under braking
- Pumping the brakes rapidly to stop on glare ice
- Charging the trailer system to full reservoir pressure
The service line to the trailer ruptures. When will you notice an effect?
- Immediately, as the trailer brakes apply
- Only when the trailer-supply valve is closed
- Never, the tractor protection valve seals it
- Only when you make a brake application
You drive a tractor without a trailer and it has no dead-end couplers. What should you do with the gladhands?
- Let them hang free behind the cab so they drain
- Tape over the openings with duct tape until needed
- Lock the service and supply gladhands together
- Disconnect them from the tractor
How should you connect gladhands?
- Clean them, put the seals together at 90° and snap down
- Line them up straight and twist clockwise
- Connect them with the engine stopped and tanks empty
- Grease the rubber seals heavily first
What colour and shape is the trailer-supply valve control usually?
- A red octagonal button
- A blue round knob
- A green square switch
- A yellow diamond-shaped button
What does a bobtail proportioning relay valve do when no trailer is attached?
- Raises service pressure to the drive axle for shorter stops
- Lowers service pressure, so you press the pedal farther
- Shuts off the front brakes completely
- Applies the spring brakes automatically
Which two valves make up the tractor protection system?
- The relay valve and the quick-release valve on the axle
- The foot valve and the trailer hand control valve
- Trailer-supply valve and tractor protection valve
- The safety valve and the governor
At what supply-line pressure does the tractor protection valve open its service-line port?
- 60 psi
- 20 psi
- 45 psi
- 100 psi
Tractor and 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 ↗
Ch. 6 Tractor protection system (pp. 31–33)
- The trailer-supply valve and the tractor protection valve form the tractor protection system, which keeps the tractor's air if there is no trailer or the trailer breaks away; it must activate at no less than 20 psi.
- Trailer-supply valve: usually a red octagonal dash button; it controls the tractor protection valve and charges the trailer reservoirs through the supply line. Also called the emergency valve.
- It is fed through a two-way check valve from the higher-pressure circuit.
- Automatic types close when pressure drops to about 20 psi; some buttons do not pop out, so confirm the spring brakes applied.
- Manual types have NORMAL and EMERGENCY positions and must be moved to EMERGENCY by hand.
- Rapid air loss in the supply line causes "dynamiting", an emergency application of the trailer brakes.
- The tractor protection valve opens its service-line port once supply-line pressure reaches 45 psi.
- The supply line holds the same pressure as the highest-pressure circuit when the supply valve is open; the service line holds air only during a brake application.
- Hand valve ("spike"): applies only the trailer brakes; used for coupling and uncoupling, and gently to help straighten a skidding tractor — never when the trailer skids; never as a parking brake.
Gladhands, bobtail valve and trailer system (pp. 34–36)
- Two-way check valves let the trailer get a higher application than the tractor when both hand and foot valves are used.
- Gladhands: clean them, join seals at 90° and snap down; use dead-end couplers or lock service and supply gladhands together when bobtailing; secure lines so they do not bounce.
- Bobtail proportioning relay valve reduces service pressure when no trailer is attached, shortening stops; the pedal must be pushed farther.
- The trailer reservoir has a draincock; the relay emergency valve relays air to trailer chambers, makes an emergency application if the supply line parts, and has a check valve so trailer air cannot flow back.
- Test the relay emergency valve by closing the supply valve or disconnecting the supply line with the valve open.
Applications and failures (pp. 38–50)
- Foot application: trailer brake pressure equals foot valve control pressure, minimizing lag.
- Trailer brakes must never hold an unattended parked vehicle.
- Breakaway: supply line loss triggers the trailer emergency application, the supply valve closes, the tractor keeps its air.
- Service line rupture: nothing happens until you brake; a held application then bleeds tractor air until the protection system closes and the trailer goes into emergency.
- Supply line rupture: trailer brakes apply at once and the supply valve closes; tractor air shuts off immediately or bleeds to at least 20 psi.
- Loss of supply reservoir air: check valves keep primary and secondary air for a limited number of stops.
- Spring-brake trailers have a trailer spring-brake valve that isolates front and rear service reservoirs, ignores slow supply leaks, and applies spring brakes after a rapid supply loss.
Study the Tractor and trailer air systems rules as flashcards →
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