Brake specialist Ferodo has issued some important advice on brake fluid testing and replacement.  The company says that brake fluid needs to be replaced every two years, in compliance with manufacturer’s recommendations, regardless of vehicle use, however this is often overlooked. 

Brake Fluid Testers such as the one manufactured by Ferodo can indicated when fluid needs replacing. This uses a probe to directly measure the boiling temperature unlike many testers which rely on conductivity testers. Especially with new generation brake fluids conductivity test are not very accurate and this can compromise the correct operation of crucial safety-related components.  

Replacement intervals are dictated by the fluid’s hydroscopic quality, which is its tendency to absorb moisture, possibly caused by a brake hose cracking or by the normal degeneration of the quality of the fluid. For example, silicone-based DOT5 is more hygroscopic than DOT3 or glycol-based DOT4. Under harsh braking, the liquid in contact with the rubber surfaces and the calliper pistons may achieve elevated temperatures, up to 200°C. Due to moisture retention, water vapour is produced, which can be compressed. Therefore, the stroke of the pedal becomes dangerously long, with a consequent worsening of the braking efficiency. One year is enough for the fluid boiling temperature to decrease by up to 80°C. For this reason, the minimum boiling points of the more common fluids are as follows:  DOT3, 205°C; DOT4, 230°C; DOT5, 250°C; DOT5.1, 260°C. It should be considered that when the DOT and the boiling point increase, the moisture retention tendency grows as well. Therefore, a DOT5.1 should normally be replaced every 6 months. 

Ferodo also recommends after several thousand KM, topping off the brake fluid level and says that a technician should never mix glycol-based DOT3 and DOT4 fluids with silicon based DOT5 fluids, as they are not compatible.

More details and advice on brake fluid replacement procedures are available on the FM-Campus YouTube channel, through the video "How to check and replace your brake fluid”.