What supplies compressed air to the brake reservoirs while the engine is running?
Correct answer: The air compressor.
The compressor pumps air into the reservoirs. The other components transmit braking force or hold the brakes, rather than replenish stored air.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Air compressor; printed and physical PDF page 107; reviewed September 20, 2026. ↗
The air pressure reaches the compressor governor’s cut-out setting. What should happen?
Correct answer: The compressor stops pumping air into the system.
The governor controls pumping at the cut-in and cut-out settings. Cut-out does not mean the engine stops or the reservoirs are emptied; use the vehicle’s specified pressure settings.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Governor cut-out; printed and physical PDF page 107; reviewed September 20, 2026. ↗
Your compressor has its own oil supply rather than sharing engine lubrication. What additional maintenance check is needed?
Correct answer: Check that compressor’s oil level.
Some compressors have a separate oil supply. An acceptable engine oil level does not establish that a separately lubricated compressor has enough oil.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Separate compressor lubrication; printed and physical PDF page 107; reviewed September 20, 2026. ↗
Why can an air-braked vehicle still have several brake applications available after the compressor stops pumping?
Correct answer: The reservoirs hold a supply of compressed air.
Reservoirs store braking air. That reserve is limited: it does not make continued driving with a failed air supply safe.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Stored-air reserve; printed and physical PDF page 107; reviewed September 20, 2026. ↗
Why is water left in an air-brake reservoir especially dangerous in freezing weather?
Correct answer: It can freeze and interfere with brake operation.
Moisture can freeze in the system and cause brake trouble. The handbook calls for draining water and oil from tanks using the equipment’s proper procedure.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Water in air tanks; printed and physical PDF page 107; reviewed September 20, 2026. ↗
Your vehicle has manual air-tank drains. What does the Wisconsin CDL handbook advise?
Correct answer: Drain the tanks at the end of each working day.
Manual drains remove collected water and oil. Waiting for a malfunction allows contamination to build up; follow the vehicle’s drain procedure and maintenance instructions.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Manual reservoir drains; printed and physical PDF page 108; reviewed September 20, 2026. ↗
A truck has an alcohol evaporator for its air system. How should it be treated?
Correct answer: Maintain it as specified; it does not eliminate tank-drain checks.
An alcohol evaporator is fitted only on some systems. Follow the equipment’s instructions and continue required moisture-management checks; do not improvise additives for other systems.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Alcohol evaporator equipment; printed and physical PDF page 108; reviewed September 20, 2026. ↗
The air reservoir’s safety relief valve opens because system pressure is too high. What does this indicate?
Correct answer: A system problem needs attention by a mechanic.
The safety valve protects against excessive pressure. Its operation is a reason to find the fault, not disable the protection or assume the brakes are otherwise sound.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Safety valve discharge; printed and physical PDF page 108; reviewed September 20, 2026. ↗
In normal air-brake operation, pressing the brake pedal farther generally does what?
Correct answer: Applies more air pressure to the service brakes.
The foot valve meters service-brake pressure according to pedal demand. It does not adjust the governor’s pressure setting.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Foot valve pressure; printed and physical PDF page 108; reviewed September 20, 2026. ↗
When you release the air-brake pedal after a normal application, what happens to the air used for that application?
Correct answer: It is exhausted from the brake chambers.
Releasing the pedal vents the application air so the service brakes can release. New air must be supplied from the reservoirs for later applications.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Releasing the service pedal; printed and physical PDF page 108; reviewed September 20, 2026. ↗
Why should you avoid pumping air brakes unnecessarily?
Correct answer: Repeated applications can use air faster than it is replenished.
Each application consumes air. Repeated unnecessary applications can reduce the available reserve, especially if the compressor cannot keep up.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Repeated unnecessary pumping; printed and physical PDF page 108; reviewed September 20, 2026. ↗
What actually slows the wheel when a drum-type service brake applies?
Correct answer: Friction between the brake lining and the drum.
Air pressure operates the brake mechanism, but contact between lining and drum creates the friction that slows the wheel and produces heat.
Source: Wisconsin Commercial Driver's Manual, May 2026; Section 5; Drum brake friction; printed and physical PDF page 108; reviewed September 20, 2026. ↗
