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What is the difference between DOT3 and DOT4 brake fluids?

What could possibly go wrong if I use a (4 times cheaper) DOT3 brake fluid instead of the DOT4 which manufacturer recommends (but allows adding "some" DOT3 occasionally)?

What will happen if I just use only DOT3?

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6 Answers 6

up vote 9 down vote accepted

If the manual is saying small amounts of DOT3 can be used, what they are most likely saying is if you find yourself with low brake fluid and only DOT3 is available, it is better to use that than to not have brake fluid.

Once you get back home though you need to get the recommended DOT4 back in the system by bleeding the system and filling with DOT4.

As already stated, DOT4 handles higher heat. If your car is calling for DOT4, that means that the manufacturer does not feel comfortable that the braking system will not raise the brake fluid above a temperature that DOT3 can handle.

Another point to make here is there are two boiling temperatures for brake fluid, Dry and Wet.

When you've just replaced your brake fluid and the system has been properly bled, you are working at the Dry boiling temperature. Over time, water works its way into the system through age, heat cycling, through the hoses, etc. You are then working at your Wet boiling point for the fluid.

So again, if you get stuck in a spot where you need to put some new brake fluid into the system and all you have available is DOT3, most likely at that point your DOT4 has degraded some and the fresh DOT3 will be close to where the DOT4 is at, but this won't stay true, the DOT3 will degrade once in the lines so the above statement of replace it as soon as possible holds true.

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Check "Brake Fluid" on Wikipedia.

DOT 5 is not interchangeable or compatible with DOT 3, 4, and 5.1 fluids and can cause catastrophic system failure.

Dot 3, 4, and 5.1 are glycol ether based. They are compatible, but like motor oils, you should use the recommended or higher grade fluid. Dot 4 and 5.1 also have borate ester to handle higher temperatures. DOT 3, 4, and 5.1 fluids are found in most brake and clutch systems.

DOT 5 is a silicone oil based fluid and can only be used in new, dry systems. It is found in racing, US Military, Soviet, and Finnish systems to handle low temperature, excessive braking, and water exposure problems.

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DOT 4 also absorbs less water than DOT 3.

(sorry Larry) DOT 5 IS compatible with DOT 3 & 4 systems but old fluid should be completely flushed out with DOT 5.

DOT 5 is NOT compatible with anti-lock brakes!

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I’ve rejected the edit, because it changes the post completely, and make it contradict within itself. If the post is wrong, it should be downvoted and that should be substantiated with references (same goes for author). The edit may well be a correct one, but there is no reference, and it is better suited as a comment. –  theUg Aug 25 '13 at 2:31
    
I would have made my comments as a comment but at the time there was no "add comment" link visible to me at the time (or now--other than the one to add this comment). I am sorry, I didn't realize we have to provide references with our answers. My point was that 5 can be used in SYSTEMS that were originally designed for 3 or 4. I will reference myself in that I have had 5 in a car that was wet with DOT 4 for over twenty years without the catastrophic fail cited above. –  twleblanc Mar 14 at 0:21

Viscosity of DOT4 is more than DOT3 and it maintains its fluidity at a higher temp. Also, the brakes shall be more effective when the system gets hot during a long drive.

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Most people realize that brakes work by turning kinetic energy into heat. The brake rotors and pads get very hot, this heat is transferred to the brake fluid as well, if the brake fluid gets too hot and boils it effectively puts air in the lines and will lead to brake failure.

You can put DOT 4 in place of Dot 3 but not the other way around.

Note DOT 5 is completely different and is silicone based, it is not compatible with DOT 3 and 4 systems

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DOT4 brake fluid has a higher boiling point than DOT3, making the fluid less likely to boil. Using cheaper, lower-grade fluid increases the chances of your brakes failing in situations where they build up lots of heat, e.g., driving down a mountain on a twisty road.

More information: http://www.carbibles.com/brake_bible_pg2.html

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