Got a technical question? Ask TRW’s expert

This month, TRW’s Brian Newell shares his thoughts on a couple of braking issues

Q: My alloy wheels are always covered in black brake dust. Is this the result of poor quality pads being fitted?

A: Possibly, but most of the brake dust comes from brake discs. With any disc and pad combination, the dust is a result of wear on both parts, but analysis of the black dust shows it contains a high percentage of steel. Good quality brake pads contain very little steel; what little there is in the material is a powder known as green steel. But discs are made of steel, so all you can do is clean the alloys once a week, sorry!

Q: My three-year-old car has just had its first MOT and the garage tried to ‘con’ me into having the brake fluid changed. So long as the level is topped up regularly, why would it need changing?

A: Brake fluid is designed to transfer forces within the brake system, and is a hydraulic fluid, not oil. All modern brake fluids are hygroscopic - they absorb water and are designed to have a dry boiling point (no water content) of around 230 - 260°C. This is effectively the thermal resilience of the fluid and it needs to be very high to combat the extreme temperatures experienced at the brake cylinder or caliper. If the fluid were to actually boil, vapour bubbles would appear and the braking performance would be severely degraded. The worst-case scenario would be brake failure.

The wet boiling point of brake fluid is around 155 - 180°C. This is the effective boiling point after the fluid has absorbed some water, as it would in normal use. It is this measure that is used when testing old brake fluid and why we at TRW together with every car manufacturer would recommend that brake fluid be changed every two years, regardless of the mileage covered.