Beyond the "PASS": Why Your Brake Test Could Be a Compliance Trap
By Zed Aziz CMILT, Transport Consultant
In my years of consulting, I’ve seen one recurring mistake that puts O-Licences at risk more than almost any other: The "Pass/Fail" Fallacy.
Many operators receive a Roller Brake Test (RBT) printout, see the word "PASS" at the bottom, and file it away. But for the Traffic Commissioner (TC), that "PASS" is only the beginning of the story. If you aren't analysing the data behind that result, you might be driving toward a regulatory disaster.
The "Pass on Locks" Illusion
On an MOT, an imbalance over 30% is a fail. However, if both wheels "lock" (stop turning) on the rollers, the machine automatically grants a pass. This is a trap.
Imagine Axle 2 on your trailer locks at 2200kg on the Nearside and 1450kg on the Offside. That is a 34% imbalance. The computer says "PASS," but in a real-world emergency stop, that trailer will pull violently to one side. If you ignore this, you are ignoring a mechanical defect.
The Step-by-Step RBT Risk Assessment
To ensure your fleet remains truly compliant, I recommend every transport manager follows this 4-step risk assessment for every printout:
Is the vehicle laden to at least 65% of its design weight? An unladen test (where wheels lock at low forces like 400kg) is effectively useless for diagnostics. The TC expects to see those brakes working under pressure.
Look past the "PASS." If any axle shows an imbalance over 20-25%, it requires action. A 25% imbalance is a "lazy" brake that is one cold morning away from becoming a 30% failure.
Compare the two systems. If the service brake is imbalanced but the parking brake is fine, you likely have a pneumatic issue (air lines/valves). If both are imbalanced, it’s a mechanical issue (shoes, drums, or adjusters).
This is the "Golden Thread" of compliance. Annotate the RBT sheet, attach the mechanic’s job card showing the repair, and ideally, perform a voluntary re-test. This proves you are an active, professional operator.
Mechanical vs. Pneumatic: A Quick Cheat Sheet
| Symptom | Likely Cause | Action |
|---|---|---|
| High Imbalance (Both Systems) | Seized adjuster, oil on drum, or worn pads. | Mechanical Stripdown |
| High Imbalance (Service Only) | Leaking air chamber or kinked hose. | Pneumatic Pressure Test |
| High Ovality (>5%) | Warped disc or "out of round" drum. | Replace Drum/Disc |
Compliance isn't about avoiding failures; it's about managing risk. Next time you see a "Pass on Locks," ask yourself: "Is this vehicle actually safe, or is it just technically legal?"