Online Browsing Platform ISO 4925:2020 - Road vehicles -- Specification of non-petroleum-base brake fluids for hydraulic systems". www.iso.org. DOT 4 brake fluid absorbs moisture via the brake lines. As you keep pressing the brake pedal, the fluid in the system warms up. Source: Pixabay DOT4: DOT4 brake fluid has a dry boiling point of 230 degrees Celsius and a wet boiling point of 155 degrees Celsius. Remember, these are only the minimum standards. Brake fluid manufacturers can and often do improve on these figures and it is possible to find DOT 4 brake fluid with a higher boiling point than some DOT 5.1 fluids on the market.

The shelf life of glycol-based DOT brake fluid is very poor due to its hygroscopic properties, which is why every bottle should come with an air tight foil liner covering the opening. As soon as this air tight seal is broken it will start absorbing moisture from the environment and the boiling point will begin to drop. Because a hydraulic system is sealed and there are no cables to stretch, it offers more reliable braking. The fluid held in the lever reservoir effectively tops the system up over time as the brake pads wear down, so that braking is consistent.DOT 5.1 is totally different than DOT 5 brake fluid. Think of 5.1 as a glycol based DOT 4 fluid that meets DOT 5 silicone-based fluid standards. The 5.1 fluids are used primarily in vehicles equipped with ABS brake systems. Now let’s cover the last type of brake fluid which is the DOT 5.1 brake fluid. But why is this fluid called 5.1, is this some kind of an update to the DOT 5 brake fluid? Well, actually no.

Hydragas and Hydrolastic suspension were a widely used form of hydropneumatic suspension, designed by Alex Moulton, and used on British Leyland cars from the 1960s. This system was not engine-driven and did not involve the braking system. There are four types of braking fluid: DOT 3, DOT 4, DOT 5.1, and DOT 5. As kinetic energy is converted to heat by brakes, brake fluids are subjected to a fair degree of heating. Source:Apel USA Quality standards refer to a brake fluid's "dry" and "wet" boiling points. The wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several (single digit) percent, varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life.On the other hand, brake fluid DOT 5 is a silicone-based product, and it should not be mixed with other fluids. This braking fluid was designed primarily for military vehicles. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5 and mineral oil based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life. Ideally, silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol-based fluid (DOT 3/4/5.1) will contain moisture; glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climates, particularly in Russia and Finland.