DOT three and DOT 4 Brake Fluid: What'due south the Difference?

We pause down the differences between DOT 3 and DOT 4 brake fluid.

The primary deviation betwixt DOT 3 and DOT four brake fluid is their respective boiling points.

I doubtable I know your next question.

Only first, some background. The U.Due south. Department of Transportation (DOT) classifies restriction fluid into four primary categories:

  • DOT 3
  • DOT 4
  • DOT 5
  • DOT v.1

Their primary differences are their moisture & dry humid points and their composition.

Dry Humid Signal Wet Boiling Point Limerick
DOT 3 205°C/401°F 140°C/284°F Glycol Ether
DOT 4 230°C/446°F 155°C/311°F Glycol Ether/Borate Ester
DOT 5 260°C/500°F 180°C/356°F Silicone
DOT 5.ane 260°C/500°F 180°C/356°F Glycol Ether/Borate Ester


DOT 3 is the most common type used in cars and trucks today. DOT four, however, is gaining popularity due to widespread use of anti-lock braking systems and traction command, which do good from DOT 4 fluid'southward lower viscosity.

Are DOT 3 and DOT four Brake Fluid Uniform?

Yes. DOT 3 brake fluid is uniform with DOT 4 brake fluid. However, DOT iv offers a higher humid point.

DOT 5.1 is used in high-performance and heavy-duty applications due to its high boiling point. It's uniform with DOT three and DOT 4 fluid.

DOT 5 brake fluid is silicone, significant it doesn't absorb water. It's non compatible with the other restriction fluids and is used mostly in classic cars that remain in storage for long periods and demand a restriction fluid that doesn't absorb water.

Bring it to a Boiling Indicate

And then, now nosotros're dorsum to humid point. What does information technology hateful? After all, we're not cooking this stuff.

Well, in the correct operating and ambience conditions, y'all are cooking it.

Glowing red brake rotor from estrus

Ambitious braking can lead to intense heat, which can vaporize the brake fluid, leading to a spongy pedal.

Braking generates intense oestrus between the brake pads and rotors. Maybe you lot've seen a race on TV where the producers stick a GoPro under the motorcar to show the brakes literally glowing ruby when the driver depresses the pedal.

Intense heat tin vaporize brake fluid, causing it to get compressible, which leads to a spongy feeling when you utilise the brakes.

That leaves gas in the lines, which is compressible, leading to a soft pedal. In racing and operation-driving circles, this is known as brake fade, and it'due south something drivers actively want to avoid. To bulldoze as finer and safely equally possible, the driver must exist confident the brakes volition perform on lap 10 as they did on lap ane.

Restriction fade can too come up from the brake pad/rotor interface. The pads release gasses, which reduces contact betwixt the pads and rotors. That'due south why loftier-end rotors are slotted and drilled – to release gasses quicker, limiting fade.

Brake Fade Isn't Just for Racers

Descending a steep hill, specially when hauling a heavy load or towing a trailer, can generate tremendous oestrus if you lot ride or pump the brakes.

Past the time you lot reach the bottom, your pedal may go nearly to the floor, making your middle rate go nearly through the roof.

PRO TIP: Adjacent time, downshift into a lower gear before descending a steep incline.

If you similar to toss your vehicle around a curvy state route for a little therapy, standing on the brakes going into corners can create sufficient heat to cause brake fade, too. If you become a little too zealous, you lot may end upwards going right through a corner and into the forest.

The fluid'southward boiling point indicates the temperature at which the brake fluid vaporizes. The college the DOT classification, the higher the boiling signal, thus the amend the fluid is at resisting heat. That's why racers employ DOT iv, not DOT 3, brake fluid.

Humid Signal is Separated Into Dry out & Wet Boiling Points

The dry humid indicate is determined using fresh fluid straight from a new container. The fluid'south moisture boiling point is adamant using fluid that'south been contaminated with 3.7% h2o, thus it's ever lower than the dry boiling bespeak. Why would test administrators contaminate practiced fluid? Because it's a reflection of what happens in the real globe.

Brake fluid is hygroscopic, meaning it absorbs h2o (except silicone-based DOT 5 restriction fluid). DOT iii fluid, for case, tin can blot up to 2% water every twelvemonth. Moisture tin enter the organisation when you lot remove the reservoir cap to add fluid, through worn seals and fifty-fifty through the safety brake lines themselves. Thus the fluid's wet boiling betoken is the number that more accurately represents what'due south really going on in your vehicle.

Since brake fluid tin wear out, it'south vital y'all change it periodically. Otherwise, not just will your brakes get spongy and unsafe, the moisture will slowly corrode metal components.

A practiced rule of thumb is to change the brake fluid every other twelvemonth in rider vehicles, and at least every yr in racing vehicles. The AAA says 88% of motorists overlook brake maintenance, and then you're not lone if you haven't changed brake fluid in awhile, similar since you bought your vehicle.

It's not as well late to first, though. And when y'all practise, bank check out our line of brake fluids for your vehicle.

Lookout man At present: How to Bleed Brakes

More like this