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A short comment on brake fluid that floated over the computer screen of our technical correspondent Dave Scott go him thinking of this ‘cinderella’ of braking systems. He turned his thoughts into action in this article.
 

Isuzu truck engineer, Frans Schellingerhout - "Moisture, as you know, is brake fluid's worst enemy as the two almost work like magnets! I have also experienced that moisture in brake fluid will corrode inside the master cylinder and then, apart from this negatively impacting the performance of the brakes, one would inevitably be faced with an expensive replacement. Wheel cylinders are certainly not excluded from this overhaul.

Due to the hygroscopic properties of brake fluid, motorists should change the lifeblood of their cars’ braking systems regularly," said Castrol Technology Manager Robbie Bowen, adding – "If they don't, they could be driving vehicles with dangerously-impaired braking ability." A brake fluid introduction in Castrol’s product guide also carries a wry comment that ‘if the engine or gearbox fails, the vehicle stops. If the brakes fail it doesn’t.’ But, what of trucks – with all those un-loader valves dumping air and hissing air leaks when stationary, do trucks have brake fluid?

The majority of trucks on our roads are equipped with vacuum/hydraulic, or air/hydraulic braking systems. Full air brakes only start to kick in at around 14t gross vehicle mass (GVM) and upwards. And because brake fluid is directed to the wheel hubs and foundation brakes, it will come into contact with massive heat generation and thus must have a high boiling point. That’s why water is brake fluid’s enemy. Hydraulic brake systems rely on ‘in-compressibility’ as an efficiency factor, and if it boils it becomes a gas and compressible. Excess water in air/hydraulic brakes weakens the brake system to transmit force and thus partially disable brakes with increased reaction times.

At an SA Guild of Motoring Journalists advanced driver course, I observed a physical stopping distance test on a Merc C55 AMG. This is some car, with all the ‘hotand- cold-running’ alphabetic acronyms – ESP (Electronic Stability Programme) ABS (Anti-lock brakes) BAS (Brake-force assist) EBD (Electronic Brake-force distribution). The test was conducted on a dry, hard surface with superb low-profile radials and a professional driver instructor who has impeccable reaction times. In a crash-stop and at 120kph the best stopping distance the C55 AMG achieved was 55m. So a 7,5t (approximately 3,5t payload) truck at 120kph hasn’t any hope of achieving near 55m – 100m at least. No wonder there are so many rearend collisions. Rear-end shunts are by far the biggest single accident factor, especially at night when poor lamps on dipped beam only reach 45m at best. Now add brakes that react poorly to pressure and the problem compounds even further.

Why is everyone ‘DOTTY’ about brake fluid? The U.S. Department of Transportation (DOT) issues brake fluid specifications. The three main types of brake fluid now available are DOT3, DOT4 and DOT5. Both DOT3 and DOT4 are glycol-based fluids while DOT5 is silicon-based. Even when heated, Glycol based fluids are apparently 2 X less compressible than silicone type fluids. The main difference is that because glycols are hygroscopic, they absorb water from the atmosphere, reducing the fluid boiling point while breaking down hydraulic efficiency. DOT3 and DOT4 absorb water – DOT5 doesn't. It’s reported that since 1982 the US Army has successfully used silicone brake fluid exclusively.

Here are a few specs and service intervals:

Toyota South Africa
Recommended Hydraulic fluid:DOT 3 Or DOT 4
Vehicles serviced @ 10 000km
Intervals: Hydraulic fluid to be checked/Replaced at 40 000km`s
Vehicles serviced @ 15 000km
Intervals: Hydraulic fluid to be checked/Replaced at 60 000km`s

DaimlerChrysler – Fuso
Recommended Hydraulic fluid:DOT3 and it is recommended that it be replaced at 12 monthly intervals.

General Motors - Isuzu
Recommended Hydraulic fluid: DOT 4 - Fluid specification: GMW 3356
N-Series: Replace every 40 000km /24 months
F-Series:Replace every 48 000km / 12 months. The kilometres will now be addressed, as this range is going to 15 000km service intervals.

It appears that time is the governing factor and ‘best practice’ would be to replace brake fluid as an annual service event. According to the latest figures there were 8,179,344 registered vehicles in South Africa by 30 April 2006. A little accurate segmentation and estimates for annual usage would provide a staggering market potential for brake fluid if it was changed only once per annum. But looking at the state of roadworthiness, and our general attitude to minimizing stopping distance reaction times, I doubt whether the annual brake fluid sales are anywhere near the potential of a market that was even 20% conscious of the need to change brake fluid regularly. It’s not speed that kills. It’s being unable to stop before a collision point that contributes to the 14 000 deaths on our roads – that’s the accepted figure at our Department of Transport. If you run a fleet, check that DOT 3 or DOT 4 fluid – 2 seconds can make a huge difference.
 

When air reservoirs are full of water and need to be drained as in this example, it’s a sure indicator that water will be present in hydraulic systems such as brake fluid.

This brake fluid container often gets over-sprayed when chassis repairers or truck body suppliers get too enthusiastic with paint when mounting equipment. It’s vital to keep brake fluid containers tightly sealed against dirt and atmospheric moisture.