CDL Combination Vehicles TestCombination Vehicle Air BrakesMedium
A combination vehicle's air brake system has a dual air brake system. If the front brake air pressure gauge indicates 60 psi, and the rear brake air pressure gauge indicates 100 psi, what is the most likely cause of this discrepancy?
- AA malfunctioning low air pressure warning system.
- BA restriction in the air line to the front brake reservoir.
- CA faulty air compressor governor.
- DAn over-pressurized rear brake system due to a stuck relay valve.
Show answer & explanationAnswer & explanation
Correct answer: B. A restriction in the air line to the front brake reservoir.
A significant difference in pressure between the front and rear brake systems in a dual air system, with one system being low, strongly suggests a restriction or leak preventing proper air flow to that specific reservoir or brake circuit.
Why the other options are wrong
- A. A malfunctioning low air warning system would not cause a pressure discrepancy, only fail to warn of it.
- C. A faulty governor would typically affect both primary and secondary systems, leading to consistent over or under-pressurization universally.
- D. An over-pressurized rear system is unlikely to be 100 psi while the front is 60 psi; 100 psi is a normal operating pressure, while 60 psi is critically low.
Dual Air System Pressure Discrepancy
In a dual air brake system, a significant difference in pressure between the primary (rear) and secondary (front) systems often indicates a problem in the air supply to the lower-pressure circuit, such as a leak or restriction.
- Dual systems have separate front and rear circuits.
- Pressure gauges monitor each circuit independently.
- Large discrepancies signal a fault requiring immediate attention.
Memory trick: Unbalanced gauges mean a 'Restriction' in the low-pressure line.