by John Baker
March 20, 2019

The primary difference between DOT 3 and DOT 4 brake fluid is their respective boiling points.

I suspect I know your next question.

But first, some background. The U.S. Department of Transportation (DOT) classifies brake fluid into four main categories:

DOT 3
DOT 4
DOT 5
DOT 5.1

Their primary differences are their wet & dry boiling points and their composition.
Dry Boiling Point Wet Boiling Point Composition
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 5.1 260°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 4, however, is gaining popularity due to widespread use of anti-lock braking systems and traction control, which benefit from DOT 4 fluid’s lower viscosity.
Are DOT 3 and DOT 4 Brake Fluid Compatible?

Yes. DOT 3 brake fluid is compatible with DOT 4 brake fluid. However, DOT 4 offers a higher boiling point.

DOT 5.1 is used in high-performance and heavy-duty applications due to its high boiling point. It’s compatible with DOT 3 and DOT 4 fluid.

DOT 5 brake fluid is silicone, meaning it doesn’t absorb water. It’s not compatible with the other brake fluids and is used mostly in classic cars that remain in storage for long periods and need a brake fluid that doesn’t absorb water.
Bring it to a Boiling Point

So, now we’re back to boiling point. What does it mean? After all, we’re not cooking this stuff.

Well, in the right operating and ambient conditions, you are cooking it.
Glowing red brake rotor from heat
Glowing red brake rotor from heat
Aggressive braking can lead to intense heat, which can vaporize the brake fluid, leading to a spongy pedal.

Braking generates intense heat between the brake pads and rotors. Maybe you’ve seen a race on TV where the producers stick a GoPro under the car to show the brakes literally glowing red when the driver depresses the pedal.

Intense heat can vaporize brake fluid, causing it to become compressible, which leads to a spongy feeling when you apply the brakes.

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That leaves gas in the lines, which is compressible, leading to a soft pedal. In racing and performance-driving circles, this is known as brake fade, and it’s something drivers actively want to avoid. To drive as effectively and safely as possible, the driver must be confident the brakes will perform on lap 10 as they did on lap one.

Brake fade can also come from the brake pad/rotor interface. The pads release gasses, which reduces contact between the pads and rotors. That’s why high-end rotors are slotted and drilled – to release gasses quicker, limiting fade.
Shop AMSOIL Synthetic Brake Fluid
Brake Fade Isn’t Just for Racers

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

By the time you reach the bottom, your pedal may go nearly to the floor, making your heart rate go nearly through the roof.

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

If you like to toss your vehicle around a curvy country road for a little therapy, standing on the brakes going into corners can create sufficient heat to cause brake fade, too. If you get a little too zealous, you may end up going right through a corner and into the woods.

The fluid’s boiling point indicates the temperature at which the brake fluid vaporizes. The higher the DOT classification, the higher the boiling point, thus the better the fluid is at resisting heat. That’s why racers use DOT 4, not DOT 3, brake fluid.

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Boiling Point is Separated Into Dry & Wet Boiling Points

The dry boiling point is determined using fresh fluid straight from a new container. The fluid’s wet boiling point is determined using fluid that’s been contaminated with 3.7% water, thus it’s always lower than the dry boiling point. Why would test administrators contaminate good fluid? Because it’s a reflection of what happens in the real world.

Brake fluid is hygroscopic, meaning it absorbs water (except silicone-based DOT 5 brake fluid). DOT 3 fluid, for example, can absorb up to 2% water every year. Moisture can enter the system when you remove the reservoir cap to add fluid, through worn seals and even through the rubber brake lines themselves. Thus the fluid’s wet boiling point is the number that more accurately represents what’s really going on in your vehicle.

Since brake fluid can wear out, it’s vital you change it periodically. Otherwise, not only will your brakes become spongy and unsafe, the moisture will slowly corrode metal components.

A good rule of thumb is to change the brake fluid every other year in passenger vehicles, and at least every year in racing vehicles. The AAA says 88% of motorists overlook brake maintenance, so you’re not alone if you haven’t changed brake fluid in awhile, like since you bought your vehicle.

you left out his closing remark, the sales pitch...

"It’s not too late to start, though. And when you do, check out our line of brake fluids for your vehicle."

https://blog.amsoil.com/dot-3-...hats-the-difference/

That said, you're going to have to do much better than a sales pitch for Amsoil written by one of their bloggers, a NOBODY, who isn't a racer nor a chassis builder of any kind much less a bracket racer, no mind a bracket racer that uses his brakes before the stripe, regurgitating well published boiling points, assumptions and opinion about brake fluids written by others for over 40 years.

This message has been edited. Last edited by: 1320racer,

Now for the FACTS per The Federal Motor Vehicle Safety Standards (FMVSS)

DOT 5 is a silicone-based fluid that must be purple

DOT 5 is immiscible with water, and with other brake fluids, and must not be mixed with them.

DOT 5 contains at least 70% by weight of a diorgano polysiloxane.

DOT 5 is hydrophobic, meaning it can maintain an acceptable boiling point over the fluid's service life. An advantage over other forms of brake fluid is that silicone has a more stable viscosity index over a wider temperature range.

DOT 5 brake fluid is not compatible with anti-lock braking systems.

DOT 5 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 standardized on DOT 5 brake fluid since the 1990s.

More 1320 Keyboard Racer/Special EDiot regurgitations.

Now, if the type of brake fluid is as big of an issue that Keyboard Racer makes it out to be. Surely NHRA would put this in the rulebook. If you're that passionate about DOT 5 fluid, go full Karen on NHRA about it.

"Use of DOT 4 or 5.1 fluid with a high boiling point and lubrication for seals and pistons is recommended. Do not use (DOT 5) silicone fluids." Source: Mark Williams Enterprises

"Use a quality DOT 4 high temp brake fluid. Silicone DOT 5 fluid is not recommended." Source: Moser Engineering

As stated before, I'll use what the brake manufacturer recommends!

2BKING

[/QUOTE]

Says a clueless broke dyck idiot^^^ who put a pro charger on his junk and can’t run better than high 9s!

and I don’t care what moser says, they are the 3rd choice at best for brakes with professional chassis builders…FACT!

This message has been edited. Last edited by: 1320racer,

1320 Keyboard F...tard, you left out the Gold Standard!

"Use of DOT 4 or 5.1 fluid with a high boiling point and lubrication for seals and pistons is recommended. Do not use (DOT 5) silicone fluids."
Source: Mark Williams Enterprises

2BKING

This message has been edited. Last edited by: B KING,

Can anyone tell me exactly what the problem with dot5 silicone fluid is? I’m building a new car and brakes using Strange brakes. Even they can’t tell me why they don’t recommend it, just they don’t recommend it. Reading msds and recommendations on applications it appears that dot5 would be the best choice provided proper maintenance is kept. These cars are aggressively driven then put in storage sometimes for weeks. That’s follows the recommendation for dot5 fluid. So, again, what exactly is the problem?

Of course they cant because they too are regurgitating myth, rumor and hearsay first stated by someone 40 years ago that didn’t have a clue, maybe an engineer or manufacturer of only DOT3 at the time. Regardless, There is no issue, no problem and you are exactly correct, EVERYTHING about what we do with these cars is EXACTLY what DOT5 was intended to do. Use it without any concern or fear.

quote:

Originally posted by Wooley:
Can anyone tell me exactly what the problem with dot5 silicone fluid is? I’m building a new car and brakes using Strange brakes. Even they can’t tell me why they don’t recommend it, just they don’t recommend it. Reading msds and recommendations on applications it appears that dot5 would be the best choice provided proper maintenance is kept. These cars are aggressively driven then put in storage sometimes for weeks. That’s follows the recommendation for dot5 fluid. So, again, what exactly is the problem?

The 1st sentence might be a clue.

"Use of DOT 4 or 5.1 fluid with a high boiling point and lubrication for seals and pistons is recommended. Do not use (DOT 5) silicone fluids."
Source: Mark Williams Enterprises

2BKING

Not looking for an argument just real facts. Btw the assembly grease that Bendix you know them one of the biggest suppliers to oem. Anyway that assembly grease for caliper seals that they supplied is silicone based. So an educated assumption can be made that lubricants are not the problem.

While I've never had an issue with DOT 4 fluid, I'll be changing to DOT 5.1

2BKING

quote:

posted June 30, 2023 10:27 PM Hide Post
by John Baker
March 20, 2019

The primary difference between DOT 3 and DOT 4 brake fluid is their respective boiling points.

I suspect I know your next question.

But first, some background. The U.S. Department of Transportation (DOT) classifies brake fluid into four main categories:

DOT 3
DOT 4
DOT 5
DOT 5.1

Their primary differences are their wet & dry boiling points and their composition.
Dry Boiling Point Wet Boiling Point Composition
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 5.1 260°C/500°F 180°C/356°F Glycol Ether/Borate Ester

DOT 5 and DOT 5.1 both have same boiling point so use which ever one you want. Does not make any difference. I went with the 5.1 simply because did not have to worry about getting all the old fluid out and all the manufacturers say not to use the DOT 5.

Seriously though this is pretty cool you can use what ever you want no matter what the Karen says.

quote:

Originally posted by Wooley:
Not looking for an argument just real facts. Btw the assembly grease that Bendix you know them one of the biggest suppliers to oem. Anyway that assembly grease for caliper seals that they supplied is silicone based. So an educated assumption can be made that lubricants are not the problem.

EXACTLY! I’ve given you the real FACTS.

If you want to discuss this further away from the clueless noise, PM me.

For the 2 clueless idiots replying here that think they know..

DOT5.1 will to overheat, cook, burn and come out BLACK in our bracket cars after just one season just like DOT3 and 4 IF you actually use your brakes…FACTS and TRUTH!

Here’s my take on the situation based on the reading I have done. The “we do not recommend dot5 fluid”, is a CYA by the manufacturer. Reason being, the manufacturer has no control over the installation or end user. Manufacturing also knows that mixing dot5 with dot3/4/5.1 will result in brake loss or failure. The manufacturer knows most people will be put off by the price of dot5 fluid and just use whatever is cheapest that can be had at the time. That’s my opinion on it and yes I know what it’s worth, no need to remind me. I was looking for an actual reason but I myself cannot find it.

This^^^

As I have stated and demonstrated every weekend, every season for decades, there is no reason not to use DOT5 and every reason to use it in our cars.

quote:

Originally posted by B KING:


1320 Keyboard F...tard, you left out the Gold Standard!

"Use of DOT 4 or 5.1 fluid with a high boiling point and lubrication for seals and pistons is recommended. Do not use (DOT 5) silicone fluids."
Source: Mark Williams Enterprises

2BKING

quote:

Originally posted by B KING:
While I've never had an issue with DOT 4 fluid, I'll be changing to DOT 5.1

2BKING

I'll stick with the so-called Gold Standard & other well-known brake manufactures recommendations; DOT 5.1 it's!

I will not take a 1320 Keyboard Karen/F...tard recommendation, especially when it contradicts his own Gold Standard Mark Williams "Do not use (DOT 5) silicone fluids."

2BKING

quote:

Originally posted by Wooley:
The “we do not recommend dot5 fluid”, is a CYA by the manufacturer. Reason being, the manufacturer has no control over the installation or end user.

Quoted for the TRUTH and the FACTS!

Wooley…KNOWS!

DOT 4 will get flushed/replaced with DOT 5.1 & used the rest of the season; then get flushed/replaced with 5.1 at the end of the season.

^^^ I KNOW! ^^^

2BKING