On a transit bus, what is the primary function of the air compressor governor?
Air Supply And Service Systems
Question · Easy
Typical air-brake governor cut-out pressure on a transit bus is approximately:
Chapter 1: Air Supply Systems & Pressure Control
1.1 Air Compressors, Governors, & Unloader Mechanisms
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The reservoir safety (pressure relief) valve in a bus air system is generally set to open at about:
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Technician A says the air dryer removes moisture and oil before air enters the supply reservoir. Technician B says the air dryer's purge cycle is triggered when the governor unloads the compressor. Who is correct?
Question · Medium
Water and oil collecting in the supply reservoir of a transit bus most directly indicates a problem with the:
What is the purpose of the one-way check valve located between the supply (wet) reservoir and the primary/secondary reservoirs?
1.2 Air Dryers, Coalescing Filters, & Moisture Control
A transit bus uses a dual air-brake system. The PRIMARY circuit normally supplies the:
The low-air-pressure warning device on a bus must activate at no less than approximately:
A bus builds air from 0 to cut-out very slowly (over 6 minutes at high idle). The MOST likely cause is:
Question · Hard
During a static air-leakage test with the engine off and brakes released, allowable pressure drop for a single vehicle is generally no more than:
1.3 Air Reservoirs, Safety Relief Valves, & Drain Valves
With the engine off and the service brakes fully applied, the allowable air-pressure leak-down rate on a single vehicle is generally no more than:
The treadle (foot/application) valve on a transit bus does which of the following?
What is the main purpose of a relay valve in a bus air-brake system?
A quick-release valve in the front service-brake circuit primarily improves:
1.4 System Check Valves & Pressure-Protection Valves
FMVSS 121 establishes maximum allowable times for air-brake systems to:
Under FMVSS 121, brake application (actuation) time is measured from the start of pedal application until brake-chamber pressure reaches:
A bus has slow brake release at the rear axle only. Technician A says a defective relay valve could trap chamber air. Technician B says a restricted exhaust port in the relay valve could be the cause. Who is correct?
The pressure-protection valve fitted to accessory air circuits on a bus is designed to:
Chapter 2: Service Brake Control Circuits & Pneumatic Valves
2.1 Dual Treadle (Brake Application) Valves
On a transit bus equipped with a door interlock, the brake interlock typically:
Excessive oil carryover passing through the air dryer into the reservoirs is MOST often caused by:
The bus compressor will not unload at cut-out and pressure keeps climbing until the safety valve blows. The MOST likely cause is:
A double-check valve in a bus air system is used to:
2.2 Relay Valves & Quick-Release Valves
On a bus that uses a brake-pedal-mounted treadle valve, a delivery-port air leak heard only when the pedal is applied indicates:
Why are transit buses typically fitted with multiple air reservoirs (supply, primary, secondary)?
An automatic drain valve on a bus supply reservoir is intended to:
A bus shows water freezing in air lines during winter despite a working air dryer. The FIRST item to check is the:
2.3 Transit Brake Valves & Safety Interlocks
The supply (wet) reservoir on a bus is so named because it:
During an applied-pressure test, the gauge drops rapidly only when the brakes are applied and is steady when released. The leak is MOST likely in the:
Which component most directly converts compressed air into mechanical force at the foundation brake?
A bus has a low-air buzzer that never sounds even at low pressure. Technician A says a failed low-pressure switch could be the cause. Technician B says a blown buzzer fuse or open circuit could be the cause. Who is correct?
Chapter 3: Air System Testing, Brake Performance Testing, & FMVSS 121 Timing
3.1 FMVSS 121 System Performance & Timing Standards
On a relay-valve-equipped rear axle, brakes apply late and weakly while the front axle responds normally. The MOST likely cause is:
Which test best evaluates whether the bus air system meets FMVSS-style reservoir build-up performance?
A purge of air and a brief 'pop' heard from the air dryer each time the compressor cuts out is:
The function of a brake (proportioning/inversion) valve sometimes used on a bus front axle is to:
3.2 Air Buildup, Recovery, & Leakage Testing
Technician A says the supply reservoir should be drained or auto-drained regularly to remove water. Technician B says a saturated air dryer desiccant can let water reach the tanks. Who is correct?
A continuous loud air leak at the dash treadle valve only when the pedal is fully released suggests a leaking:
The governor is mounted on or near the compressor and is connected to it by a:
A bus loses all air pressure rapidly with the engine off and brakes released, dropping far faster than 2 psi/min. The technician should FIRST:
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