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Combination Vehicles Practice Question 29

Required for Class ACombination Vehicles · Question 29

A leaky air line between tractor and trailer will most likely cause:

• AHigher braking force
• BLoss of trailer brake function and possible automatic application
• CFaster acceleration
• DImproved fuel economy

Correct answer: B. Loss of trailer brake function and possible automatic application

Why: Air leaks reduce trailer brake function and may trigger automatic emergency brake application by the tractor protection valve. Repair leaks before driving.

How this question fits the Combination test

Articulated vehicles fail in unique ways: trailer swing under braking, rearward amplification under steering, and jackknife under loss of traction. Combination Vehicle knowledge is required for any Class A applicant under 49 CFR 383.111.

Study technique for this topic

Walk through the coupling sequence verbally until it's second nature. Know the safe-rolling rearward amplification factor, the difference between trailer service brakes and parking brakes, and what creates the conditions for a tractor jackknife.

Related Combination practice questions

• Q25. The 'safety latch' on the fifth wheel locking lever:
• Q39. The forward thrust between trailer and tractor on a hard stop is absorbed by:
• Q52. A tractor-trailer encountering a steep downgrade should choose a gear:
• Q1. A combination vehicle is generally defined as:
• Q14. The first sign of a tractor drive-wheel skid is often:

Continue your CDL preparation

The Combination Vehicles portion of the CDL knowledge exam covers more than this single question. Use this site's full Combination Vehicles bank to drill every topic, then test yourself with one of our state-specific practice sets to confirm you are ready for the actual DMV exam in your state. Always check your state's CDL handbook for any local addenda before the test day.

If you are still building your overall CDL foundation, start with the General Knowledge bank — every CDL applicant must pass it — and then work outward to the endorsements you plan to add.