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Quizzes > General Knowledge & Mixed

You Supply Air to Trailer Tanks: CDL Air Brakes Quiz

Quick, free CDL Air Brakes quiz on trailer tanks air supply. Instant results.

Editorial: Review CompletedCreated By: Hieu NguyenUpdated Aug 26, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration showing Free CDL Air Brakes quiz banner with trailer tanks valves gauges on dark blue background

This quiz helps you practice how to supply air to trailer tanks and avoid common mistakes before your CDL exam. For more study, try the air brake system test, see why spring brakes held back matters, and sharpen hookups with the coupling and uncoupling practice test.

What is the primary purpose of the trailer supply valve on a tractor?
To open the fuel tank for refueling
To regulate engine RPM
To adjust transmission pressure
To control air flow into the trailer air tanks
The trailer supply valve (often the yellow knob) allows air from the tractor system to fill the trailer tanks. It is essential for supplying and isolating trailer brakes. Closing this valve protects the tractor system if trailer piping develops a leak. .
At what minimum air pressure should you open the trailer supply valve to charge trailer tanks?
100 PSI
45 PSI
65 PSI
150 PSI
Most air brake systems are designed to start charging trailer tanks when tractor pressure reaches at least 100 PSI. This ensures enough reserve pressure in both tractor and trailer tanks before adding load. Underpressurized systems risk insufficient brake application. .
What color is typically used to identify the trailer supply line (glad-hand) on a tractor unit?
Blue
Green
Red
Yellow
The trailer supply line is usually marked with a yellow glad-hand or color-coding to distinguish it from the service line. Consistent color coding prevents misconnections. This is a standard practice in commercial air brake systems. .
What does the tractor protection valve do when trailer air lines develop a major leak?
Keeps air flowing to trailer tanks
Closes to protect tractor air tanks from loss
Increases air pressure to compensate
Switches supply to emergency reserves
The tractor protection valve closes automatically if pressure drops too low or if the trailer line is damaged, preventing loss of air in the tractor's reservoirs. It protects essential truck brakes and steering from losing pressure. Repairs must be made before resuming normal operations. Air Brake Valve Functions.
When are spring brakes automatically applied on a trailer?
Only when parked on a hill
When service brake is applied
If trailer air pressure falls below a threshold
When glad-hands are disconnected
Spring brakes use powerful springs that apply when air pressure drops below a certain level (usually around 60 PSI) or if the supply is intentionally released. They act as emergency and parking brakes. This failsafe prevents uncontrolled rolling. .
What is the proper way to connect tractor and trailer glad-hands?
Staple each line to the frame
Twist lock then push air line in
Push together and secure retainer clips
Screw into place like a hose fitting
Glad-hands are rubber-cushioned couplings that you push together and lock with a metal retainer clip. They seal air lines between tractor and trailer. Proper connection prevents air leaks and brake failures. .
Which gauge on the tractor dashboard shows trailer air pressure?
Trailer supply gauge
Primary (front) tank gauge
Secondary (rear) tank gauge
Oil pressure gauge
After the trailer supply valve is opened, the trailer supply gauge displays pressure in the trailer reservoirs. Monitoring this gauge ensures proper brake readiness. Using the wrong gauge could lead to misunderstanding system status. .
When should you check for air leaks after connecting trailer tanks?
When brakes feel spongy
Only after driving 5 miles
Immediately after charging to full pressure
At the end of the day
You must test for air leaks right after the system is charged to ensure no fittings or hoses are leaking. This check is part of pre-trip inspection. Undetected leaks may cause brake failure. .
What steps do you take to test that air is properly supplied to the trailer tanks?
Park on hill, apply trailer parking brake, release trailer glad-hands
Disconnect and reconnect service line three times
Charge system, pull trailer emergency valve, check leak rate
Close tractor protection valve, drive forward, check service brakes
Proper testing involves charging the system to full pressure, then pulling the trailer emergency (red) control to ensure spring brakes apply. Checking the leak rate verifies system integrity. This verifies both service and emergency supply. .
What is the function of the poppet in the tractor protection valve?
Regulates governor cut-in pressure
Allows air to flow to the trailer when open and shuts when closed
Activates ABS at low speed
Filters moisture from air
The poppet inside the tractor protection valve moves to allow or block air flow to the trailer tanks. If pressure drops or the valve is shut, the poppet seals to protect the tractor system. Proper poppet function is critical to air brake safety. .
If trailer brakes fail to release after charging air, what is a likely cause?
Service line is leaking or disconnected
Emergency spring brakes are stuck
Trailer supply valve is not fully opened
Tractor engine RPM too low
If the trailer supply valve isn't fully opened, air cannot flow continuously into the trailer reservoirs to reset the spring brakes. Other causes may include a frozen brake chamber but the most common is an under-opened valve. Checking the valve position solves this. .
What are normal governor cut-in and cut-out pressures for most air brake systems?
Cut-in 120 PSI, cut-out 140 PSI
Cut-in 60 PSI, cut-out 80 PSI
Cut-in 100 PSI, cut-out 120 PSI
Cut-in 80 PSI, cut-out 100 PSI
Most air compressors will cut in at about 100 PSI and cut out at around 120 - 125 PSI. This ensures a safe pressure window for brake operation and reserve capacity. Deviations indicate governor or compressor issues. .
Which control on the dash is used to supply air to the trailer tanks?
Service brake pedal
Yellow trailer air supply knob
Red tractor protection valve
Parking brake control
The yellow knob on the dashboard controls the trailer air supply valve. Depressing or pulling it opens or closes air flow to the trailer. The red knob controls the emergency spring brake function. Correct control avoids confusion. .
What sequence should you follow when coupling a new trailer?
Raise tractor, connect glad-hands, charge trailer, check for leaks
Secure trailer brakes, then back in, then plug in lines
Connect electrical first, then glad-hands, then secure pintle hook
Charge trailer first, then park tractor, then connect lines
Correct sequence: Secure both units, back tractor under the trailer, connect glad-hands, open supply valve to charge the trailer, listen for leaks, and verify pressure build-up. This prevents damage and ensures safe coupling. .
If the service line to the trailer leaks, what happens when you apply the brakes?
Nothing happens; service line is not used
Trailer brakes will lock up
Trailer brakes release but tractor brakes remain
Trailer brakes may not apply, causing longer stopping distance
A leaking service line prevents adequate air from reaching the trailer actuators on brake application, meaning less or no trailer braking. This increases stopping distance dangerously. Identifying and repairing leaks is mandatory before operation. .
What is the purpose of a double-check valve in an air brake system?
Reduce pressure for parking brakes
Filter moisture from air lines
Combine pressure from two sources and protect against failure of one
Automatically charge trailer tanks
A double-check valve allows air from either of two sources (e.g., primary and secondary reservoirs) to feed a single circuit while protecting against loss if one source fails. It ensures critical functions have backup pressure. .
After coupling and charging, trailer tanks will not build past 25 PSI. What is the most likely cause?
Brake chambers are frozen
Trailer service line is disconnected
Tractor protection valve not opening
Governor set too low
If trailer tanks only charge to very low pressure, the tractor protection valve poppet may be stuck closed or the valve not fully opened. This restricts air flow into the trailer. Inspect the trailer supply valve and poppet mechanism. .
At what pressure do spring brakes typically apply automatically on most tractors and trailers?
90 - 110 PSI
60 - 80 PSI
45 - 60 PSI
20 - 40 PSI
Spring brakes are usually designed to apply automatically when air pressure in the system drops to around 60 PSI. This threshold ensures the vehicle won't roll if air pressure is low. Check your system specs for exact values. .
How do you drain moisture and oil from trailer air tanks during pre-trip inspection?
Disconnect glad-hands and let system dump
Open the drain cocks at the bottom of the tanks after air is drained
Run engine at idle for 10 minutes
Activate service brakes twice
Trailer tanks have manual drain cocks at the bottom. You must open them after reducing system pressure, allowing built-up moisture and oil to escape. Regular draining prevents corrosion and brake malfunction. .
Why might a mechanic use the trailer supply test port instead of the service line when diagnosing issues?
Service port bypasses tractor protection valve
Supply port isolates trailer reservoir pressure
Service port measures governor cut-in
Supply port filters out contaminants
The trailer supply test port taps directly into the trailer reservoir, bypassing service and emergency lines. This lets you accurately measure trailer tank pressure and diagnose leaks or valve failures. It's crucial for pinpoint diagnosis. .
How do partial stroke or modulation valves in some trailer systems affect air supply?
They bypass governor function
They conserve air by limiting stroke to release service brakes slowly
They increase pressure faster
They apply spring brakes automatically
Modulation valves (partial stroke) help conserve air by adjusting the release stroke of spring brakes during service applications. They control the rate of stroke to save air without full brake release. This technology improves efficiency. .
What impact do ABS modulator valves have on trailer air supply during antilock events?
They switch valves to secondary reservoir
They cut off trailer supply immediately
They increase spring brake pressure
They momentarily reduce service line pressure to prevent wheel lockup
ABS modulator valves rapidly modulate service line pressure at each wheel to prevent lockup, momentarily reducing brake cylinder pressure. This does not affect supply to trailer reservoirs but controls brake application pressure. .
How does the emergency supply valve sequence air flow when engaged in a dual trailer setup?
Diverts all air to the second trailer first
Shuts off supply entirely until manually reset
Supplies first trailer then bypasses to second once first is charged
Equally splits pressure between both trailers simultaneously
On dual trailers, the emergency supply valve is designed to charge the first trailer reservoir completely before allowing air to fill the second. This ensures emergency brakes on the lead trailer are fully set. Only after the front trailer is charged does the second receive air. .
How does the spring brake hold-back mechanism operate during trailer supply loss?
It holds trailer spring brakes off until pressure drops below a secondary threshold, preventing unintended application
It automatically releases tractor spring brakes
It disables ABS until service pressure returns
It closes the governor to reserve air
Spring brake hold-back valves delay spring brake application until service pressure drops below a preset secondary threshold, ensuring minor fluctuations don't cause emergency stops. They improve ride quality while maintaining safety. This mechanism is specific to certain heavy combinations. .
In a fully charged tractor-trailer system, what is the correct sequence of air flow through valves when initially supplying trailer tanks?
Governor ? Trailer Supply Valve ? Compressor ? Trailer Reservoirs
Compressor ? Trailer Supply Valve ? Tractor Protection Valve ? Trailer Reservoirs
Compressor ? Governor ? Tractor Reservoirs ? Tractor Protection Valve ? Trailer Supply Valve ? Trailer Reservoirs
Compressor ? Trailer Reservoirs ? Tractor Reservoirs ? Governor
Air leaves the compressor, passes through the governor, fills the tractor reservoirs, then flows through the tractor protection valve when open, through the trailer supply valve, and finally into the trailer reservoirs. This ordered path ensures proper pressure regulation and emergency protection. .
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Study Outcomes

  1. Supply Air to Trailer Tanks -

    Apply the correct procedure to you supply air to the trailer tanks by using the tractor protection valve, trailer air supply knob, and monitoring gauge readings to achieve proper pressure before driving.

  2. Explain Air vs. Hydraulic Braking Response -

    Explain why air braking takes more time than hydraulic braking because air compresses, affecting brake response and requiring drivers to anticipate stopping distances.

  3. Identify Spring Brake Holdback -

    Describe how during normal driving spring brakes are usually held back by air pressure, preventing premature activation and ensuring consistent braking performance.

  4. Use Parking Brakes Properly -

    Demonstrate when and how with air brake vehicles the parking brakes should be used to secure parked rigs and prevent unintended movement.

  5. Assess Front Wheel Brake Performance -

    Evaluate why front wheel brakes are good under all conditions, delivering reliable stopping power and stability across various road surfaces.

  6. Master Air Brake System Operations -

    Apply the knowledge gained from the quiz to confidently inspect, test, and operate air brake systems ahead of the real CDL Air Brakes exam.

Cheat Sheet

  1. Supplying Air to Trailer Tanks -

    When you supply air to the trailer tanks by pushing in the trailer air supply control, you open the tractor protection valve and allow compressed air to fill the trailer reservoirs from the tractor's supply lines. Always confirm a full charge by watching the green "supply" light and listening for the air build-up, which typically takes around 45 - 60 seconds in a dual-circuit system. Mnemonic: "Pull In, Press Out, Tanks Fill with a Shout!" helps you remember the correct push - pull sequence.

  2. Air vs. Hydraulic Braking -

    Air braking takes more time than hydraulic braking because air must be compressed before pressure builds enough to actuate the brake chambers, so drivers need to anticipate stops early and maintain larger following distances. The compressibility of air adds a slight delay, meaning fully loaded rigs can require 20 - 40% more stopping distance compared to hydraulic systems. Pro tip: add one second of following distance for every ten feet of vehicle length at speeds above 40 mph.

  3. Spring Brake Holdback Mechanics -

    During normal driving spring brakes are usually held back by air pressure acting against a powerful spring in the brake chamber, preventing the brake shoes from engaging until service brake pressure drops. If air pressure falls below 20 - 45 psi, the spring forces expand, automatically applying the brakes for emergency or parking functions. Remember the phrase "Air In, Spring Free; No Air, Spring Stops Me" to recall how spring-brake holdback works.

  4. Proper Use of Parking Brakes -

    With air brake vehicles the parking brakes should be used every time you park by pulling the yellow control knob out, which vents the air and lets the spring brakes engage mechanically, securing your vehicle. Always wait until system pressure drops and the dash indicator confirms spring-brake activation before exiting the cab. This vital step is emphasized in FMCSA regulations and most state CDL manuals to prevent dangerous rollaways.

  5. Front Wheel Brake Application -

    Front wheel brakes are good under all conditions thanks to weight transfer during braking, which increases traction on the front axle, but over-applying them can still cause wheel lockup on slick surfaces. Apply pressure smoothly and avoid sudden, hard pushes; a handy mnemonic is "Squeeze, Don't Snatch" to maintain control and prevent skids. According to SAE research, balanced brake usage optimizes stopping distance and vehicle stability.

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