As part of your pre-trip inspection of a combination vehicle with air brakes and one trailer, you should make sure that the air-loss rate per minute is not greater than
- 3 psi.
- 1 psi.
- 2 psi.
- 4 psi. ✓
Answer: D. 4 psi.
For a combination vehicle with air brakes and one trailer, the air-loss rate after the initial pressure drop should not be more than 4 psi per minute.
Why this is the answer
The handbook's Class 1 pre-trip inspection tells you to build up a full brake application of 90 to 100 p.s.i. and hold it for a minute with the engine off, so you can hear leaks. After the first pressure drop, the loss must stay at or below 4 p.s.i. (27.6 kPa) per minute for a vehicle pulling one trailer, and 6 p.s.i. with two trailers. That is the limit you check against during your inspection, along with confirming there are no audible air leaks.
Why the other answers are wrong
- A. 3 psi. — Three p.s.i. per minute is the figure the handbook gives for the Class 3 pre-trip check, not for a Class 1 combination unit with a trailer.
- B. 1 psi. — One p.s.i. per minute is far tighter than any leak limit the handbook lists for a pre-trip air-loss check.
- C. 2 psi. — Two p.s.i. per minute is not a limit that appears in the handbook; the allowance for one trailer is higher than that.
Remember it
Remember the trailer count as a ladder: one trailer gives you 4 p.s.i., and a second trailer doubles it to 6.