Brake Fluid Guidance

Brake Fluid should be replaced every 2 years and is not mileage dependant, meaning even if you have driven your car for 1 year or more the fluid should still be replaced every 2 years, why?

OK, brake fluid is hygroscopic which in laymen terms means it absorbs water from the atmosphere, this absorption deteriorates the fluid by lowering the boiling point of the brake fluid it can also cause an increase in volume within your brake system causing complete brake lockup in rare circumstances. The lower the boiling point of the brake fluid the more likely the chances of vapour lock when this occurs the fluid boils separating the oxygen from the water (just like boiling your kettle) the oxygen now in the brake system greatly reduced the hydraulic efficiency of your brake system, this will be felt as a “Spongy Pedal” feel and you will also notice the brake don’t work as well as they should. This is because the pressure in your hydraulic system being applied by your brake master cylinder / brake pedal is being absorbed by the oxygen, the reason being oxygen is compressible and when confined behaves almost spring like where as brake fluid is designed not be compressible. A very easy way to understand this is, if you had a solid steel bar and pushed from one end parallel to the bar the other end would move at exactly the same timing and distance, however if you divided the solid bar and placed small balloons filled with air between each pocket and carried out the same test, the bar at the very front would not move immediately and would not travel the same distance as the rear.

The most common type of brake fluid is DOT4 but there is also DOT 3, DOT5 and finally DOT5.1 these fluids are different and its important that you understand why, these can be separated into 2 groups,

  1. Glycol (polyethylene glycol) Based
  2. Silicone (diorgano polysiloxane) Based

Glycol based fluids are the hygroscopic type which means they absorb water as discussed above, where as Silicone based fluids contain at least 70% by weight of a diorgano polysiloxane when replacing DOT 5 fluid extra care should be taken as this fluid does absorb air this is known as hydrophobic the most critical point however is Silicone based fluids and Glycol based fluids should NOT be mixed.

Below is the common boiling points for the fluid types.

Type Dry boiling point
DOT 3 205 °C (401 °F) 140 °C (284 °F)
DOT 4 230 °C (446 °F) 155 °C (311 °F)
DOT 5 260 °C (500 °F) 180 °C (356 °F)
DOT 5.1 260 °C (500 °F) 180 °C (356 °F)

Available from our stock are DOT  4 and DOT 5.1, we have standard replacement Mintex DOT 4 available in half litre and 1 litre’s and for the DOT 5.1 we have Ferodo racing fluid, which can be used on the road and track.

Contact Info

Unit 1 Fan Road, Chesterfield, Derbyshire ,S43 3PT

01246 281543
sales@brakedepot.co.uk

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