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Spongy brakes? It could be vapour lock

By autotech-nath on July 27, 2017

Has a customer ever complained of brakes that feel spongy or offer little resistance? This is potentially a sign of vapour lock, a dangerous phenomenon that can result in a complete loss of braking. It is caused by the trapping of gas in the brakes’ hydraulic system. Fluid testing is vital to determine whether contamination has occurred and if vapour lock has compromised the performance of the braking system. We spoke to brake specialist Ferodo to gain some insight into how it can be avoided, diagnosed and corrected.

The process of braking generates a large amount of heat, which is why brake fluids are designed with high boiling points, ensuring they remain liquid even when heated. Unfortunately, brake fluid is also hygroscopic, which means that it absorbs moisture. Over time, the infiltration of moisture reduces the brake fluid’s boiling point.

“Once the concentration of water reaches a certain point, the brake fluid will start to boil under braking, filling the lines with gas bubbles.”

Jonathan Allen, Regional Marketing Manager at Federal-Mogul Autoparts, explains ” As gas is compressible, the driver will feel no resistance when pressing the brake pedal and the brakes themselves will not be applied, potentially causing an accident.”

So, how do you avoid an accident?

1 – Choose the right brake fluid

The choice of brake fluid is critical to avoiding vapour lock. Always use OE-quality and adhere to manufacturer specifications.

2 – Test the brake fluid

Many vehicle manufacturers provide guide times for brake fluid replacement, but in reality, it should not be based on mileage or age. Ferodo recommends routine checks to ensure safety and generate sales opportunities. The Ferodo Brake Fluid Tester (FFT100A), uses industry standard techniques like the Karl Fisher method for water content, ensuring much more accurate measurement than ‘pen-type’ test systems.

3 – Change the brake fluid

If the test shows that the boiling point of the fluid is too low, it must be changed. A master cylinder leak past test is required, before bleeding the system. It should be pressurised to 50 bar for 45 seconds, during which time there should be no more than a 4 bar loss in pressure. If it passes the leak pass test, you should then bleed the brakes in set order, starting with the caliber furthest from the master cylinder. Once complete, check the brake pedal for adequate feel and pressure.

With regular and accurate testing, it is possible to avoid the dangers of vapour lock. It is also important to note that other losses in braking efficiency could be due to brake fade, overheating, pad degradation, faulty brake hoses, fluid leakage or hose ballooning. A thorough check of the entire system is always recommended.



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Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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