BRAKE FLUID
▶ Being the medium to transmit hydraulic pressure from the master cylinder to the pads/ shoes, the importance of brake fluid must not be underestimated. Indeed, the water-absorbing properties of the conventional synthetic glycol-based DOT 3, 4 and 5.1 specifications can cause total brake failure, if ignored. This is not just down to leaks, where corrosion within the various hydraulic cylinders abrades and wears the rubber seals, but also a lowering of the boiling point. As friction brakes turn motion energy into heat, the brake fluid must be able to withstand high temperatures, which is why fresh brake fluid boils at around 270°C. Yet, when contaminated with just 3% moisture, this boiling point can fall by as much as 100°C. Should the fluid boil within the braking system, air bubbles are formed, meaning that the forces can no longer be transmitted, because the air is compressed instead of the fluid. This vapour lock situation causes the brake pedal to sink to the floor with no corresponding retardation. Unfortunately, there is nothing you can do to stop moisture from contaminating the fluid, because it enters through the semi-permeable flexible rubber brake hoses, seals, joints and the cap breather hole.
This also means that you cannot store brake fluid indefinitely, because it can even absorb moisture from the air in your garage, when stored in a sealed bottle. The rate at which it does this depends on a range of factors, including storage conditions and the bottle material, grade and thickness. Therefore, presume a shelf life of between 18 and 24 months.
Considering this rather annoying characteristic, instead of just checking the fluid level, its condition should be assessed, too. Inexpensive conductive testers can be dipped into the reservoir, which pass a small electrical current through the fluid to gauge the water content. Yet, they are not especially accurate. One company told us that such testers might pose a danger, by passing fluid as safe, when its boiling point has been compromised severely.
A more accurate way is to collect a fluid sample and record the temperature at which it boils.
Such testers offer far more accurate readings but their cost tends to preclude their use to professional garages. Textar’s TEX100BFT costs (typically) £300+VAT, for instance, although some less expensive testers are appearing on the market. We found Sealey’s VS0275 available for less than half that outlay. Similarly, used professional bargains can be found online.
If you prefer not to test the fluid, you can still follow best practice recommendations. Brake fluid change interval recommendations vary between car manufacturers and braking specialists but they tend not to exceed once every two years. Conducting the operation tends to be much easier with a pressurised brake bleeder, which means that the entire operation can be completed by one person. Starting at the wheel furthest from the master cylinder, opening each brake nipple allows the system to be flushed with fresh fluid.
Yet, research your vehicle’s specifications, especially if it is a newer model. Some cars require that the braking system is interrogated diagnostically, to enter the brake bleeding mode. This function holds any required electro valves within the circuit open, allowing fluid to pass through.
When working, consider that brake fluid is both poisonous and injurious to paintwork. Any accidental spills should be rinsed away with water immediately.