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I've been going to a lot of muscle car websites the last few days and have seen quite a few recommendations to switch over a brake system from DOT4 to DOT5.

The work that's required seems to revolve around various seals and rubber components within the braking system. It seems it is quite involved.

Why would I want to change over to DOT5 if that much work is involved?

Are the benefits that huge where I would be motivated to do this?

What are the applications and benefits to DOT5 brake fluid?

Additionally I have seen references to DOT5.1?

From what I understand the foundation of the fluid has a different chemical composition than the other, hence the seal changes but at the end of the day it seems the benefits would have to be quite extreme in order to be motivated to do the conversion.

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For everyone's edification, here are the dry/wet boiling point specifications for the different brake fluids:

            Dry boiling point   Wet boiling point
DOT 3       205 °C (401 °F)     140 °C (284 °F)
DOT 4       230 °C (446 °F)     155 °C (311 °F)
DOT 5       260 °C (500 °F)     180 °C (356 °F)
DOT 5.1     260 °C (500 °F)     180 °C (356 °F)

Why would I want to change over to DOT5 if that much work is involved?

With the advent of DOT5.1, you probably would not want to switch over. One of the major disadvantages of DOT5 is it doesn't work with anti-lock braking systems. If you want to remove this part of your braking system when doing the change over, then you'd be okay. If you do not fully clean your braking system with solvents while switching over, the DOT5 will be contaminated and will not function as expected.

Also, if there is water left in the system during the switch over, the water will pool or "puddle" within the braking system which can cause corrosion. If that pool or puddle of water just happens to be in your caliper (or winds up there), it can flash over to steam if the environment gets hot enough, which can cause you all kinds of issues (back flow of brake fluid out of the system; complete loss of braking). This makes it very important to get all of the water out of the system when using DOT5.

Are the benefits that huge where I would be motivated to do this?

This question is very subjective, but for the most part, no. With DOT5.1 available, there really aren't huge benefits for moving to DOT5.

What are the applications and benefits to DOT5 brake fluid?

As vini_i stated, DOT5 doesn't absorb water like DOT3/4/5.1, so that is a major plus factor for it. DOT 5 has a service temperature range of -55°C (-67°F) to 55°C (131°F). It is used exclusively by the US Military for this reason. If you live in the Yukon Territories (Canada) or Norther Alaska (US), you may have an argument for using DOT5. DOT5 is also specified for some Harley Davidson motorcycles (though I understand they have stopped specifying it for their later models).

Additionally I have seen references to DOT5.1?

As alluded to previously, DOT5.1 is Glycol Ether/Borate Ester based just like DOT3/4. This makes them compatible, unlike DOT5 (which is silicone based). DOT5.1 has the same Wet/Dry boiling point specification as DOT5 as well. It will work with antilock braking systems, just like DOT3/4. Plus, you don't have to do any major disassemble of the braking system to switch over to it (as long as your braking system doesn't call for DOT5 or it wasn't previously converted over to DOT5).

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The biggest advantage to DOT5 brake fluid is that it is hydrophobic meaning it doesn't like water. This is in contrast to DOT 3, 4, 5.1 which are hygroscopic which means it will absorb water even just out of the air. This is a huge advantage if storing the car for most of the year and only breaking it out a few times during the summer. If using any other fluid periodic flushes would be required to keep the fluid moisture free and the brake system rust free. DOT5 also does not damage paint if spilled.

DOT5.1 brake fluid uses the same basic chemicals as DOT 3 and 4 but has a higher boiling point. Until DOT5.1, DOT5 had the highest boiling point of the different brake fluids. It was common to see DOT5 used in high performance applications such as racing. DOT5.1 and DOT5 have the same boiling point so now in a high performance application DOT5.1 can be used with normal brake components without worrying about the seals.

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    Dot 5's biggest advantage is also its disadvantage. Because it will not hold water, any water in the system collects on the bottom of the cylinders and can quickly cause local pitting. Dot 5 cannot be mixed with the others so switching from one to the other is a lot of work. The system most be taken apart and cleaned.
    – Fred Wilson
    Dec 29, 2015 at 3:37
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    That's probably why each manufacturer specifies which fluid should be used.
    – I have no idea what I'm doing
    Dec 29, 2015 at 11:02
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Years ago I had a 57 Chevy and every 1-2 years I had to rebuild the brake cylinders.
Leaking and freezing and the flex lines would swell up. After the introduction of Dot 5, I converted it over. 20 years later I hadn't touched the brakes again except for linings. In 95, I got a 78 Chevy 3500 truck with leaking cylinders. Replaced all hydraulics like the 57. Master cylinder, calipers, wheel cylinders and all. I just sold the truck, 25 years later, and the hydraulics were still fine!! This stuff is great for saving the expense and hassle of rebuilding the systems, especialy on cars we don't drive every day. I also labelled the M/C cap so I wouldn't forget and some clutz didn't accidently add wrong fluid.

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