June 2013

How often should my brake fluid be changed?

 by autosphere on  |
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We here at Autosphere are often asked about fluid replacement intervals, and one of the more common queries we receive is, ‘when brake fluid should be replaced?’  So we asked one of our technical contributors, Thomas Bullock to help write a short article on brake fluid change intervals, and here are his thoughts: Firstly like any fluid change interval, brake fluid is heavily dependent on the type of use the car is subject to. A street car and a track car will have very different requirements.  This article is only concerned with DOT3, 4 and 5.1 fluids, typically used in passenger cars. The primary function of brake fluid is to transfer hydraulic pressure from the pedal through the master cylinder to the individual pistons or wheel cylinders at each corner.  DOT 3, 4, and 5.1 fluids are commonly glycol ester based, and these types of fluid are all hygroscopic, or more simply, they readily absorb water from the atmosphere.  This absorption is important as it prevents any water collecting within the brake system and corroding the metal components within. However a secondary effect of this water absorption is decreasing boiling point as the fluid collects more water, which is why both wet and dry boiling points are quoted for brake fluids.  Managing this water absorption, along with other contamination and degradation of the fluid are the main reasons to change to fresh fluid.  Water enters the system via diffusion through the brake hoses and through the master cylinder breather. For a car that sees circuit use it’s important for the boiling point of the fluid to remain as high as possible, as the high temperatures reached can cause the fluid to boil in the calipers, making for a soft pedal and inconsistent performance.  So the fluid should be changed more often than for a street car, however given these cars are often pampered and see fewer street kilometers than most, this doesn’t mean that the fluid must be flushed every track event. ATE Super Blue Racing / Type 200 Amber is a DOT 4 brake fluid with a dry boiling point of 536F/280c and a wet boiling point of 392F/200c and is regarded as a bang for buck performance brake fluid when compared to the likes of Motul RBF 600 / 660 and Brembo LCF600. As a general rule, for a street/track car, we would recommend a full fluid replacement every 6 months to 1 year, with only a bleed of 250mL of fluid at most through the system before every track day or two. Ultimately though the intervals your car requires will become clear over time and track use, if for example after a number of track days the fluid boiling point has decreased enough to cause soft pedal fade issues, then more regular changes are obviously needed.  However the freshest fluid won’t solve a fundamental heat issue with brakes, so if you’re consistently cooking fluid it’s best to look for other solutions other than the fluid. Conversely for a car that solely sees street use, it is often best to simply follow the manufacturer’s recommended fluid change interval.  This is usually 1-2 years, which will see the brake fluid boiling point decrease noticeably from its dry boiling point, but still maintain an adequate level for the conditions encountered during road use.  Of course if you subject your street car to more severe conditions, such as driving in hilly terrain or towing then you should consider more regular fluid changes.  
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