Bendix joins the DOTs (on brake fluid).

News

27 Aug

When choosing replacement brake fluid, it’s important to select one that has a DOT rating that’s compatible with your truck, to ensure the braking system functions safely.

DOT refers to Department of Transport – it’s a standard that sets the safety regulations and benchmarks for different brake fluids. Brake fluids can be DOT 3, DOT 4, DOT 5 and DOT 5.1, and what you need depends on the make, model and age of the vehicle. This important information is listed in the owner’s manual and can also be advised by a mechanic or brake technician.

The DOT ratings are based on the fluid’s wet and dry boiling points. The term ‘dry boiling’ point refers to when the fluid is brand new, fresh from the sealed container, while ‘wet boiling’ point refers to fluid that’s one or two years old and has taken on some moisture and contaminants. This occurs because most brake fluid types – DOT 3, DOT 4 and DOT 5.1 – are hygroscopic, meaning they absorb water. DOT 5 however is glycol-based.

Although the braking system is considered a closed system, over time air can enter in several ways: when the cap is removed from the brake fluid reservoir, via microscopic pores in rubber hoses, through the normal driving cycle as the brake fluid heats and cools, or when the brake fluid is replaced.

Generally speaking, the higher the DOT rating, the higher the boiling point and longevity of the fluid. As a hydraulic fluid with the primary function of transferring force from pedal to brake calipers or drums, it’s critical to safe performance that brake fluid remains incompressible. At lower boiling points (caused by moisture absorption) the fluid can vaporise affecting how force is transferred, leading to a soft or spongy pedal feel and in a worst-case scenario, lead to delayed or complete loss of braking power.

Approximate wet and dry brake fluid boiling points

DOT TYPE DRY BOILING POINT WET BOILING POINT

DOT 3 205° C 140° C

DOT 4 230° C 155° C

DOT 5 260° C 180° C

DOT 5.1 270° C 190° C

DOT 4 fluids are the most widely used – Bendix offers both high performance Polyglycol DOT 3 and DOT 4 fluids, providing optimum performance in both hydraulic and conventional braking systems, meeting stringent Australian and U.S specifications.

Brake fluid ‘DOs and ‘DON’Ts’

Don’t use old/opened brake fluid bottles – fluid can become contaminated within a few months
Don’t overfill the reservoir – brake fluid expands when hot
Don’t mix incompatible DOT types. Mixing silicon with glycol-based fluids can cause a gel formation that can damage the brake system and even lead to brake failure
Don’t ignore a brake fluid leak, it’s a sign that there’s a fault within the system
Do take care to avoid spilling brake fluid on painted surfaces – it’s highly corrosive to paint
Do minimise the time the cap is off the brake fluid reservoir and seal tightly
Do change brake fluid every one to two years, or to the vehicle manufacturer’s specification