CDL Air Brakes Test

B. There is a malfunction in the ABS system.

If the ABS indicator light stays on after the engine starts, it indicates that the ABS system has detected a fault and is not functioning properly. While the regular brakes should still work, the anti-lock feature will not be available.

A. A defective parking brake control valve.

If the spring brakes aren't releasing, but there's enough air pressure in the system and the parking brake knob is 'in' (signaling release), it means the air isn't reaching the spring brake chambers or isn't being properly directed to compress the springs. The parking brake control valve is responsible for allowing air into (to release) and out of (to apply) the spring brake chambers. A defect here would prevent release.

B. A leak in the primary air tank or its associated lines/components.

In a dual air system, both tanks should typically be at or near the governor's cut-out pressure. If one tank's pressure is significantly lower (90 psi) while the other is near cut-out (120 psi), it indicates a loss of air from the lower-pressure tank due to a leak.

D. Excessive slack in the foundation brakes.

If the brake pedal requires more travel for effective braking, and air pressure is normal, it suggests that the force from the brake chamber isn't being efficiently transferred to the brake shoes. Excessive slack in the foundation brakes (e.g., improperly adjusted slack adjusters or worn components) would cause the pushrod to travel further before the shoes contact the drum, resulting in a 'long' pedal.

A. Immediately pull over and have the vehicle inspected, as this indicates a severe malfunction.

The low air pressure warning system should activate at or above 60 psi. If it activates at 55 psi, it indicates a significant air leak or a faulty warning system, both of which are critical safety concerns requiring immediate attention.

B. The brake pedal (foot) valve.

If there's a continuous air leak at the brake chambers when the brakes are not applied, it means air is constantly being sent to them. The brake pedal (foot) valve is responsible for controlling the flow of air to the service brake chambers. A leak in this valve, or if it's stuck partially open, would cause air to leak past it and out through the chambers.

C. The brake pedal (foot) valve.

If air is continuously leaking from the brake chambers when the brakes are not applied, it indicates that air is being sent to the chambers when it shouldn't be. The brake pedal (foot) valve controls the flow of air to the service brake chambers, so a fault in this valve is the most likely cause.

D. To reduce air pressure to the front brakes to prevent front wheel lock-up.

The front brake limiting valve, when set to 'wet' or 'slippery,' reduces the air pressure applied to the front brakes. This is designed to prevent the front wheels from locking up prematurely on slick surfaces, which could lead to a loss of steering control.

D. The air dryer is malfunctioning or absent.

Air dryers remove moisture and oil contaminants from the compressed air before it enters the storage tanks. The presence of moisture and oil in the tanks indicates the air dryer is not performing its function properly.

D. A restricted air intake filter on the compressor.

A restricted air intake filter starves the air compressor of air, significantly reducing its efficiency and increasing the time it takes to build pressure.

D. Worn foundation brakes (e.g., brake linings or drums).

A spongy brake pedal feel, especially requiring greater effort for braking, often indicates issues with the foundation brakes. Worn brake linings, drums, or slack adjusters can lead to excessive travel in the brake components, requiring more air pressure and pedal travel to achieve effective braking.

C. A leak in the secondary air tank system.

In a dual air brake system, if the compressor is running and the primary tank is at full pressure (120 psi), but the secondary tank is significantly lower (80 psi), it indicates that air is not being retained in the secondary system. This is most commonly due to a leak in the secondary air tank itself or its associated lines/valves.

C. Compromised structural integrity of the tank, leading to potential rupture.

Air tanks are designed to hold high-pressure air. Rust and pitting on the exterior surface indicate corrosion, which can weaken the tank's walls and compromise its structural integrity. This poses a significant risk of tank rupture under pressure, which could cause serious injury or damage.

D. A faulty slack adjuster.

If the brake chamber pushrod extends but the brake shoes don't move, it indicates a problem in the linkage between the pushrod and the brake shoes. A faulty or improperly adjusted slack adjuster is the most common cause, as it's responsible for transmitting the force from the pushrod to the cam and thence to the brake shoes.

B. Worn or damaged brake drums/rotors.

If the slack adjusters are correctly set, but braking still feels spongy or requires excessive pedal travel, it often points to issues with the foundation brakes themselves, such as worn, cracked, or out-of-round brake drums/rotors. These conditions prevent the brake shoes/pads from making full, consistent contact.

D. A faulty air compressor governor failing to cut out the compressor.

The safety relief valve opens to prevent catastrophic over-pressurization of the air tanks, typically when pressure exceeds 150 psi. If it's releasing air at 130 psi, and continuously, it indicates that the air compressor is still pumping air beyond its normal cut-out pressure (usually 125 psi) because the governor has failed to signal it to stop.

B. The stop light switch.

The stop light switch is specifically designed to activate the vehicle's stop lights when the service brakes are applied. If the brakes work but the lights don't, the switch is the most direct point of failure.

A. A stuck-open safety valve on an air tank.

If the safety valve on an air tank is stuck open, it will continuously release air, preventing the system from building up to its maximum operating pressure (typically 125 psi) even if the compressor is working.

D. To prevent moisture in the compressed air from freezing during cold weather.

Alcohol evaporators are used in older air brake systems to inject alcohol vapor into the air lines. This alcohol mixes with any moisture present, lowering its freezing point and preventing ice formation in cold temperatures.

B. Increased risk of rear-end collision.

The stop light switch activates the brake lights to warn following motorists that the vehicle is slowing or stopping. If it fails, other drivers will not be alerted, significantly increasing the risk of a rear-end collision.

C. Worn or damaged air compressor components.

The federal standard for air brake systems requires the compressor to build air pressure from 85 to 100 psi in 45 seconds (for dual systems) or 50 to 90 psi in 3 minutes (for single systems or older vehicles). An unusually long time to build pressure, especially from 0 psi, often points to internal wear or damage within the compressor itself, reducing its efficiency.

D. To prevent ice formation in the air brake lines during cold weather.

Alcohol evaporators are used in older air brake systems (before air dryers became standard) to inject alcohol vapor into the air lines. This alcohol mixes with any moisture, lowering its freezing point and preventing ice from forming and blocking the lines.

D. A continuous, small air leak in the system.

If the compressor is cycling frequently but still reaches correct pressure, it indicates that air is being lost from the system at a rate higher than normal. A continuous, small air leak would cause the pressure to drop more quickly, forcing the compressor to cycle more often to maintain pressure.

D. The slack adjusters require adjustment.

Excessive free play in the brake pedal before the brakes engage is a classic symptom of out-of-adjustment slack adjusters. Slack adjusters compensate for lining wear, and if they are not correctly adjusted, more pedal travel is needed to move the pushrod far enough to apply the brakes.

B. The spring brakes are engaged.

In commercial vehicles with air brakes, pulling out the parking brake control knob releases air pressure to the spring brakes, allowing the springs to apply the parking/emergency brakes. Pushing the knob in charges the spring brakes with air, releasing them.