The operation of vehicles is prohibited if:

Explanation

If the pneumatic brake system pressure gauge does not work.

In a pneumatic brake system the stopping force is supplied by compressed air, and the pressure gauge is the driver's only window onto the state of that system. If the gauge does not work, you cannot tell whether the reservoir is full or draining; a slow leak or a compressor failure proceeds silently, and the driver only discovers at the first serious braking that he does not have enough pressure. For a vehicle weighing tens of tonnes, that means a stopping distance no street can accommodate. That is why a faulty pressure gauge on its own prohibits operation.

The first item is a numerical trap. The figure 1.6 mm is lodged in everyone's mind as "the tread limit", but that limit applies to passenger cars. For a lorry the minimum residual tread depth is 1 mm; so 1.6 mm on a lorry is above the limit and entirely lawful. The question-setter is counting on precisely this substitution.

Memorise the table once and for all: passenger car 1.6 · lorry 1 · bus 2 · motorcycle 0.8 mm. In the exam, read the figure and the vehicle type together; the figure alone means nothing.

And this chapter's constant rule: the "in both cases" option is invalidated by one sound item.

• 1.6 mm is the limit for a passenger car, not a lorry; for a lorry the minimum is 1 mm, so this tread is lawful and no offence. • Since the first item is not a defect at all, "in both cases" cannot be correct; only the faulty pressure gauge creates the prohibition.

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