What’s most likely to cause a crash, if it’s allowed to get too low?

Explanation: You should carry out frequent checks on all fluid levels. The brake-fluid level will fall as the friction material on your brake shoes or pads wears down. If it falls below the minimum mark on the fluid reservoir, air could enter the hydraulic system, and this will lead to loss of braking efficiency.

Category: Band 2 traffic signs and signals

References: Highway Code: Page 130