Brake Fluid Explained: DOT Ratings, Boiling Points and Why It Ages

Brakes work because fluid does not compress. Vapour does. Everything about fluid specification comes back to keeping it liquid at the temperature your brakes actually reach.

What the DOT numbers mean

DOT 3, DOT 4 and DOT 5.1 are all glycol based and mix with each other. Each rating sets a minimum dry boiling point, measured fresh, and a minimum wet boiling point, measured with 3.7 percent absorbed water. DOT 3 requires 205 degrees Celsius dry and 140 wet, DOT 4 requires 230 and 155, and DOT 5.1 requires 260 and 180. DOT 5 is the odd one out: it is silicone based, does not absorb water, and must not be mixed with the others or used in most systems designed for glycol fluid. Use what the manufacturer specifies as a minimum, and note that a higher rating is not automatically better if the fluid absorbs water faster.

Why the wet number is the real one

Glycol fluid pulls moisture out of the atmosphere through hoses, seals and the reservoir cap. After a couple of years in service, several percent water content is normal, and it is the wet boiling point that describes the fluid in your car. Water also promotes internal corrosion of steel lines, cylinders and ABS components, which is the slow damage nobody sees. This is why the interval is time based rather than mileage based, typically every two years on most passenger cars. A cheap tester or a look at the fluid colour gives a rough idea, but the interval is simpler to follow.

Fade from boiling fluid

There are two different long pedal complaints. Pad fade is a loss of friction at the disc and the pedal still feels firm. Fluid fade is compressible vapour in the lines, and the pedal goes long and soft, sometimes recovering after a cooling lap. Old fluid brings that second failure forward, which is why a fluid change is the first thing to do before a track day and often does more than fitting expensive pads. High temperature racing fluids buy a much higher dry boiling point but absorb water quickly, so they suit a car whose fluid is changed often, not one left for years.

The conditions that matter

• ✓Glycol fluids DOT 3, DOT 4 and DOT 5.1 are compatible with each other.
• !Replace on a time interval, commonly every two years, because water content rises whether the car is driven or not.
• ✕Silicone DOT 5 is not a drop in upgrade and must not be mixed with glycol fluid.

Sources

These are the provisions the road-legality rules linked above rest on.

• MOT inspection manual: cars and passenger vehicles (GOV.UK)
• Vehicle insurance (GOV.UK)

General guidance only. Details vary by exact vehicle and change over time; confirm with a reputable specialist before modifying.