Borderline Numbers on a Single-Unit Pre-Trip
07:45 in Tampa. A 33,000-lb GVWR dump truck with a single rear axle and air brakes on all wheels is due out at 08:00. The driver drains the tanks, idles to governor cut-out at 122 psi, shuts down, and runs his tests, writing the numbers on his hand:
- brakes released, one minute: 121 to 119 psi
- full foot-valve application, one minute: 121 to 117 psi
- fast idle from 85 psi: reaches 100 psi in 62 seconds
- warning buzzer: sounds at 57 psi
- spring brakes: fully applied at 34 psi
He also notes the steering-axle chambers are standard-stroke clamp type and the pushrods extend about 1 7/8 inches with the brakes applied. He says, "Leaks are under 3 and 4, so I'm good," and reaches for the door handle.
1. Evaluate each of the five readings against the limits for a single-unit vehicle and say which fail.
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His limits are wrong - the 3-and-4 figures are combination limits. For a single unit: released leak, 2 psi per minute. His 2 psi is exactly at the limit, so it passes but is worth watching. Applied leak, 3 psi per minute; his 4 psi exceeds it and fails, meaning at least one chamber, hose, or valve leaks only when the treadle is down. Build-up from 85 to 100 psi must take 45 seconds or less; 62 seconds fails, so the compressor, unloader, governor, or drier is not keeping up. The warning at 57 psi passes - it must sound before 60. Spring brakes at 34 psi pass - they must apply inside the 20-to-45 band. The 1 7/8-inch pushrod is inside the 2-inch limit for a standard-stroke chamber but close enough that the slack adjusters need checking.
2. Are the slow build-up and the applied leak the same fault? Explain what a mechanic should check for each.
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Treat them as two separate faults until proven otherwise. The build-up test runs with the brakes released, so a chamber that leaks only under application is not being asked to hold air during that test and will not slow the build. A slow build points upstream: a worn or slipping compressor, a stuck unloader, a governor cutting in too low, a saturated air drier, a purge valve stuck open, or a restricted intake. The applied leak points downstream: a service diaphragm, a hose, or a valve. Both are real, and both matter. The dangerous combination is a compressor with no reserve margin plus any leak at all - on a long grade, repeated applications will pull reservoir pressure below the compressor's recovery rate and the gauges will walk down.
3. The driver argues the truck brakes fine on the road. What do you tell him, and what is your decision?
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Tell him the truck stops well right up until it doesn't. A 4-psi-per-minute applied leak plus a compressor that needs 62 seconds to make 15 psi means the air supply has no reserve. A descent or heavy stop-and-go traffic will consume pressure faster than it is replaced, and the first real symptom could be the buzzer with a loaded dump bed. Neither finding is something a driver can nurse along. The vehicle is out of service until both are repaired and the full sequence passes, including the marginal items - the 2-psi released leak and the 1 7/8-inch pushrod. Record every reading and the date on the DVIR so the shop has a baseline and so there is proof the defect was reported before the truck moved.