Air Brake Test Emergency Procedures & Safety Protocols 2 — Questions and Answers
Question 1: When air pressure drops to approximately 20–45 psi in a vehicle with spring brakes, what happens?
- The spring brakes automatically apply, bringing the vehicle to a stop (Correct answer)
- The service brakes become more powerful due to higher spring force
- The low pressure warning light turns off
- The driver must manually activate the emergency brake valve
Correct answer: The spring brakes automatically apply, bringing the vehicle to a stop
Spring brakes are held off by air pressure; when pressure falls to approximately 20–45 psi, the springs overcome the air and apply the brakes automatically as a fail-safe measure.
The mechanical springs in spring brake chambers are held compressed by air pressure. As air leaks out of a failing system, there comes a point where the spring force overcomes the remaining air pressure and the brakes apply. This automatic application is a critical safety feature ensuring the vehicle will stop even if the air system fails completely.
Question 2: What should you do if your air pressure warning light comes on while driving?
- Safely pull off the road immediately and stop before pressure drops to the point where spring brakes apply (Correct answer)
- Continue driving to the nearest repair shop without stopping
- Press the brake pedal harder to compensate for lower pressure
- Increase engine RPM to speed up pressure recovery
Correct answer: Safely pull off the road immediately and stop before pressure drops to the point where spring brakes apply
The warning light activates at about 60 psi; you must stop safely before pressure drops further and the spring brakes apply automatically, which could cause a sudden and uncontrolled stop.
When the low air pressure warning activates, there is limited time before the spring brakes apply automatically. The driver should immediately signal, check mirrors, and bring the vehicle to a controlled stop off the road. Continuing to drive risks the spring brakes applying suddenly at highway speed, which can cause loss of control or a jackknife in a combination vehicle.
Question 3: What is a 'brake fade' and how does it affect stopping distance during an emergency?
- Reduced braking effectiveness due to heat, which significantly increases stopping distance (Correct answer)
- Automatic application of all brakes during an emergency stop
- The ABS system reducing brake pressure to prevent wheel lock
- Gradual increase in brake pedal travel as linings wear
Correct answer: Reduced braking effectiveness due to heat, which significantly increases stopping distance
Brake fade occurs when friction surfaces overheat, reducing their ability to generate stopping force, which greatly increases stopping distance — especially dangerous in an emergency.
During prolonged or heavy braking, drums and linings can reach temperatures that cause the coefficient of friction between them to drop sharply. This is brake fade. In an emergency situation where maximum braking is needed, brake fade can mean that the brakes simply cannot generate enough force to stop the vehicle in time. Drivers should use proper braking technique to prevent overheating before an emergency arises.
Question 4: In an emergency where the trailer begins to jackknife, what is the correct immediate action?
- Release the brakes to allow the trailer wheels to regain traction and straighten (Correct answer)
- Apply harder brake pressure to lock all wheels simultaneously
- Steer sharply away from the direction of the skid
- Activate the trailer hand valve immediately
Correct answer: Release the brakes to allow the trailer wheels to regain traction and straighten
Releasing brake pressure allows the locked trailer wheels to roll again, restoring traction and allowing the trailer to straighten. Continued braking sustains the jackknife.
A jackknife occurs when trailer wheels lock and the trailer pushes the tractor sideways. The instinct to brake harder makes it worse. Releasing the foot brake reduces braking force on trailer wheels, allowing them to roll and regain traction. If the tractor is equipped with ABS, the system does this automatically, but older vehicles require manual pedal release to recover.
Question 5: What is the role of the trailer breakaway valve in an emergency separation?
- It automatically applies the trailer brakes if the trailer becomes disconnected from the tractor (Correct answer)
- It releases all trailer brakes when the trailer detaches
- It shuts off airflow to the tractor brakes to preserve pressure
- It prevents the glad hand connections from separating during normal driving
Correct answer: It automatically applies the trailer brakes if the trailer becomes disconnected from the tractor
The breakaway valve uses a pull cord connected to the tractor; if the trailer separates, the cord pulls the valve, cutting off air to the trailer service line and applying the trailer spring brakes.
The trailer breakaway valve has a cable attached to the tractor. In the event of an accidental separation, the cable pulls the valve which seals the service line and vents the trailer supply line, causing the trailer spring brakes to apply. This prevents a runaway trailer from causing additional accidents. The cable must be properly routed and not too tight or too loose.
Question 6: Why should a driver never attempt to 'fan' the brakes (rapidly pumping) on a vehicle equipped with air brakes during an emergency stop?
- Repeatedly pumping depletes the air supply faster, potentially causing complete brake failure before stopping (Correct answer)
- Fanning causes the ABS system to deactivate automatically
- It increases wear on the linings too quickly to stop in time
- Pumping activates the spring brakes prematurely
Correct answer: Repeatedly pumping depletes the air supply faster, potentially causing complete brake failure before stopping
Each brake application uses air; rapid repeated pumping in an emergency rapidly depletes tank pressure, potentially draining enough air to cause spring brake application before the vehicle can be stopped safely.
Unlike hydraulic brakes where pumping can build pressure, air brakes rely on stored compressed air. Each application exhausts some of that air. Rapid fanning in an emergency drains the system quickly, and if pressure falls to spring brake engagement level, the spring brakes apply suddenly and completely, potentially causing loss of control. Proper air brake emergency technique is a firm, steady application.
Air Brake Test Emergency Procedures & Safety Protocols 2
When air pressure drops to approximately 20–45 psi in a vehicle with spring brakes, what happens?