CDL Combination Vehicles TestInspecting Combination VehiclesMedium
A driver is performing a pre-trip inspection of a combination vehicle. When checking the air lines between the tractor and trailer, they notice that the rubber seal on the blue (service) glad hand is missing. What is the immediate consequence of this defect?
- AThe trailer's lights will not function correctly.
- BThe trailer's emergency brakes will automatically apply.
- CThe trailer's service brakes will not receive proper air pressure.
- DThe tractor's air compressor will not build pressure.
Show answer & explanationAnswer & explanation
Correct answer: C. The trailer's service brakes will not receive proper air pressure.
The blue (service) glad hand carries air pressure from the tractor's foot valve to the trailer's service brakes. A missing seal will cause a significant air leak, preventing the service brakes from receiving proper pressure and thus not functioning effectively or at all.
Why the other options are wrong
- A. Trailer lights are controlled by the electrical connection, not the air lines.
- B. Emergency brakes are controlled by the red (emergency/supply) line, not the blue (service) line.
- D. The tractor's air compressor builds pressure in the tractor's tanks; while a leak would cause it to work harder, it would still build pressure initially.
Glad Hand Seal Importance
Glad hand seals are critical for maintaining airtight connections between the tractor and trailer air lines, ensuring proper pressure for braking and spring brake release.
- Missing or damaged seals cause air leaks.
- Leaks in the service line affect service brake performance.
- Leaks in the emergency line affect spring brake release and emergency braking.
Memory trick: No seal, no squeeze; your brakes won't please!