Brake Test on a Light Truck with a Suspect Load-Sensing Valve
A 2015 4x2 light truck (GVM 4,500 kg) is presented for CoF. On the roller brake tester, service brake results are: front axle 62% efficiency, rear axle 38% efficiency, with a left/right imbalance on the rear axle of 24%. The vehicle is unladen. The rear axle is fitted with a load-sensing proportioning valve. The inspector notes the valve linkage appears to have been recently adjusted, with fresh tool marks on the adjuster and the linkage set to a position that would suit a loaded vehicle. Parking brake efficiency is 22%. The owner says the truck 'brakes fine when loaded.'
1. Interpret the brake test results. Which results fail, and how does the load-sensing valve explain the pattern?
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Front axle 62%: marginal to failing depending on the standard applied - typically a service brake efficiency of at least around 50% is required, so 62% may pass on efficiency but is low for a front axle. Rear axle 38%: FAIL on efficiency (below the typical minimum) and the 24% left/right imbalance is a clear fail (imbalance limits are usually around 30% on an axle, but 24% is high and should be investigated). Parking brake 22%: likely FAIL - parking brake efficiency requirements are commonly around 16–20% minimum, so 22% is marginal and should be checked carefully. Load-sensing valve: the valve has been set to a 'loaded' position, which on an unladen vehicle reduces rear braking effort - explaining the low rear efficiency. However, the valve should be set so braking is correct across the load range; a valve adjusted to favour loaded operation at the expense of unladen performance is a defect. The imbalance suggests a further issue (seized caliper, contaminated lining, or unequal valve delivery) beyond the valve setting alone.
2. What should the inspector do about the load-sensing valve and the imbalance before deciding the outcome?
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Do not simply re-adjust the valve to make the numbers pass. Steps: (1) Verify the valve linkage is correctly connected, free-moving, and set to the manufacturer's specified unladen position - the fresh tool marks suggest it was deliberately altered. (2) Investigate the 24% rear imbalance independently: check for a sticking caliper/slave cylinder, contaminated or glazed linings, restricted hose, or unequal valve output. (3) Re-test after correcting any fault, with the valve in the correct unladen setting. (4) Consider a loaded test if the vehicle's brake system is load-sensitive and the unladen test cannot demonstrate adequate performance - but the unladen test must still meet the minimum. Outcome: FAIL until the valve is correctly set and the imbalance resolved. Record the valve condition, the adjustment evidence, the imbalance figures, and the corrective action required.