Air Brake Pressure Buildup and Leak-Down on a Log Truck
A 2011 8x4 logging truck with a full air system is presented for a Certificate of Fitness. The inspector notes the following during the brake test. With the engine running at governed speed, reservoir pressure builds from 620 kPa to 690 kPa in 2 minutes 40 seconds. With the engine stopped and service brakes fully applied, pressure drops from 690 kPa to 560 kPa in 60 seconds. The low-pressure warning device activates at 430 kPa. The truck has a trailer coupled and the inspector has not yet isolated the trailer circuit. The truck's brake system is fitted with an alcohol evaporator and the compressor is belt-driven.
1. Assess the buildup and leak-down results against typical heavy vehicle air brake performance expectations. What is your pass/fail judgement and reasoning?
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Buildup: A typical heavy vehicle air system is expected to reach operating pressure (commonly around 690 kPa governor cut-out) within a reasonable time from a low starting point. Going from 620 to 690 kPa in 2 min 40 s is slow - a healthy compressor at governed speed normally achieves this in well under a minute. This suggests a weak compressor, slipping/worn belt, leaking unloader, or a significant system leak. Leak-down: A 130 kPa drop in 60 s with brakes applied is excessive. Typical acceptance is a much smaller drop (often cited as no more than around 10 kPa/min for a coupled unit, or roughly double that when a trailer is isolated, depending on the standard applied). Judgement: FAIL on both counts. Before condemning the compressor, the inspector should isolate the trailer and repeat the static leak test to localise whether the loss is on the truck or trailer. The low-pressure warning at 430 kPa should also be checked against the required actuation point.
2. The inspector wants to isolate the trailer to localise the leak. Describe how to do this correctly and what the isolated results would need to show before the truck could be considered acceptable on the towing vehicle alone.
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Close the trailer supply (emergency) line at the truck's trailer control valve or use the trailer isolation/ shut-off valve, and blank or close the trailer service coupling so the tractor's own system is sealed. With the trailer isolated, repeat the static leak test with brakes applied. The tractor alone should hold pressure far better - typically no more than about 10 kPa/min loss. If the tractor still loses pressure at the pre-isolation rate, the fault is on the truck (leaking chambers, valves, lines, or the foot valve). If the tractor holds but the coupled unit leaks, the fault is on the trailer. Important: even if the tractor passes in isolation, the truck is presented as a coupled unit and the CoF decision applies to the vehicle as presented - a leaking trailer circuit still means the combination fails unless the trailer is separately certified and the truck is being inspected only as a towing vehicle. The inspector must record which configuration was tested and on what basis the pass/fail was reached.