Air Brake Test System Components & Functions 2 — Questions and Answers
Question 1: What is the function of the relay valve in a combination vehicle air brake system?
- To speed up brake application and release at the trailer axles by responding to tractor brake line signals locally (Correct answer)
- To regulate air pressure supplied to the compressor
- To connect the tractor and trailer air lines at the fifth wheel
- To prevent air from flowing back into the supply tanks from the primary circuit
Correct answer: To speed up brake application and release at the trailer axles by responding to tractor brake line signals locally
Relay valves are located near the trailer axles and apply/release trailer brakes using local air supply rather than waiting for air to travel the full distance from the tractor, reducing brake lag.
Without relay valves, the time for air to travel from the foot valve at the tractor cab to the trailer axle brake chambers would create an unacceptable brake lag. Relay valves are positioned near the brake chambers and use a signal (control) line from the tractor to trigger the local application of air from the trailer's own supply tanks. This near-instantaneous response greatly improves brake timing and consistency.
Question 2: What does the double check valve do in an air brake system?
- It allows the highest of two incoming air pressures to pass through to operate a common component, such as stopping lights or spring brake release (Correct answer)
- It allows air to flow in two directions simultaneously
- It prevents pressure from exceeding the governor cut-out level
- It equalizes pressure between the primary and secondary circuits
Correct answer: It allows the highest of two incoming air pressures to pass through to operate a common component, such as stopping lights or spring brake release
Double check valves select the higher of two input pressures to supply a single output, commonly used so either the service brake application or the parking brake circuit can trigger the brake lights.
Double check valves have two inlets and one outlet. The higher of the two inlet pressures passes through to the outlet; the lower inlet is automatically blocked. This is used in air brake systems where either the service brake or parking brake circuit must be able to activate a common device, such as brake lights or the spring brake release circuit, without the two circuits cross-feeding each other.
Question 3: What is the purpose of the foot valve (brake valve) in an air brake system?
- To modulate and control the air pressure delivered to the service brakes in proportion to how hard the driver presses the pedal (Correct answer)
- To direct air to the spring brakes when the pedal is fully depressed
- To release air from the wet tank directly to the brake chambers
- To maintain constant pressure regardless of pedal position
Correct answer: To modulate and control the air pressure delivered to the service brakes in proportion to how hard the driver presses the pedal
The foot valve is a graduated control device that delivers more or less air to the service brakes depending on the force applied to the pedal, allowing the driver to modulate braking.
The foot valve (also called the brake valve or treadle valve) is a precision graduated valve. Light pedal pressure delivers low air pressure to brake chambers, providing gentle braking. Increasing pedal pressure delivers more air and stronger braking. Full pedal deflection delivers maximum air. This graduated response is fundamental to driver control of air brakes and prevents the jerky on/off application that a simple on/off valve would produce.
Question 4: What does the spring brake modulating valve (also called the hand control valve or inversion valve) do?
- It allows the driver to partially apply the spring brakes to control the vehicle if service brakes fail (Correct answer)
- It converts foot brake signals into hand brake applications automatically
- It prevents spring brakes from applying when service brake air pressure is lost
- It balances air pressure between front and rear axles during braking
Correct answer: It allows the driver to partially apply the spring brakes to control the vehicle if service brakes fail
The spring brake modulating valve allows manual, graduated control of spring brake application, useful as a modulated emergency brake if the service brake system fails.
In emergency situations where service brakes have failed, the driver can use the spring brake modulating (or 'maxi-brake') control to apply spring brakes in a graduated manner. This prevents the sudden full application of all spring brakes, which could cause loss of control. The valve releases air from spring brake chambers in proportion to its application, allowing modulated stopping force.
Question 5: What is the function of the drain cock (manual drain valve) on an air tank?
- To release accumulated water and oil from the tank to prevent corrosion and freezing (Correct answer)
- To manually adjust system operating pressure
- To vent air when pressure exceeds governor cut-out
- To connect the tank to the service brake circuit
Correct answer: To release accumulated water and oil from the tank to prevent corrosion and freezing
The drain cock is opened to expel water and oil that accumulate at the bottom of air tanks, preventing corrosion inside the tank and ice formation in cold weather.
Compressed air always carries some moisture, which condenses in the tanks as the air cools. Oil vapor from the compressor also accumulates. If not removed, water causes rust inside the tank and can freeze in cold temperatures, blocking air passages. The drain cock (manual) or automatic drain valve allows this accumulated liquid to be expelled. Manual drains should be opened at least daily.
Question 6: What is the difference between the primary and secondary air circuits in a dual air brake system?
- The primary circuit typically operates the rear (drive) axle brakes and the secondary circuit operates the front axle and trailer brakes (Correct answer)
- The primary circuit is for service brakes and the secondary for spring brakes only
- The secondary circuit supplies air only when the primary fails
- There is no difference — both circuits supply the same brake groups
Correct answer: The primary circuit typically operates the rear (drive) axle brakes and the secondary circuit operates the front axle and trailer brakes
In a typical dual air system, the primary circuit serves the rear axles and the secondary circuit serves the front axle; if one fails, the remaining circuit can still provide braking on those axles.
Dual air brake systems have two separate air circuits to provide redundancy. If one circuit fails (loses pressure), the other continues to provide braking on its axles. The low air pressure warning activates for the affected circuit, and the driver retains partial braking while bringing the vehicle to a safe stop. This separation between front and rear braking circuits is a critical safety design feature.
Air Brake Test System Components & Functions 2
What is the function of the relay valve in a combination vehicle air brake system?