Combination vehicle air brakes
Section 6.2 of the Florida CDL Manual (p. 6-4) explains the air brake system on combination vehicles, including the trailer lines and valves. It is tested on the Combination Vehicles test.
Key facts
| Part of | the Combination Vehicles test (80% to pass) | 49 CFR 383.135 |
|---|---|---|
| Manual section | §6.2 Combination air brakes | FL CDL Manual p. 6-4 |
| Questions in CDL Mile's bank | 19 | CDL Mile, checked against FL CDL Manual |
Key rules
- § 6.2.2p. 6-5
The tractor protection valve guards the tractor's own air supply. If the trailer breaks away or develops a bad leak, it keeps air in the tractor or truck brake system.
Tractor Protection Valve
- § 6.2.1p. 6-4
Never park using the trailer hand valve. The air can leak out, and on a trailer without spring brakes the brakes then let go. Use the parking brakes, and chock the wheels of a trailer that has no spring brakes.
Trailer Hand Valve
- § 6.2.1p. 6-4
Use the trailer hand valve only to test the trailer brakes. Using it while driving can make the trailer skid; the foot brake sends air to every brake on the rig, including the trailer's, and is much less likely to cause a skid or jackknife.
Trailer Hand Valve
- § 6.2.4p. 6-5
Color coding helps keep the lines from being mixed up. When colors are used, the service line is blue and the emergency (supply) line is red; some lines carry metal tags stamped with their names instead.
Trailer Air-lines
- § 6.2.3p. 6-5
Push the trailer air supply knob in to supply the trailer with air. Pulling it out cuts off the trailer's air and applies the trailer emergency brakes.
Trailer Air Supply Control
Try two questions
Combination Vehicles · Combination air brakes
Your trailer breaks loose from the tractor and tears the emergency air hose apart. What happens to the trailer's brakes?
Tap an answer to see the explanation and the manual page.
Combination Vehicles · Combination air brakes
Why is the trailer's service line connected to relay valves?
Tap an answer to see the explanation and the manual page.
More Combination Vehicles topics
Sources
Florida Commercial Driver's License Manual, version July 2022, Florida Department of Highway Safety and Motor Vehicles. Page references are to its printed page numbers. CDL Mile is independent and not affiliated with the FLHSMV.
Updated for the July 2022 Florida CDL manual. Last checked November 5, 2026.
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