C1 DOT 3 or DOT 4 brake fluid?

[Rod K.]

Ready to "fluidize" my '57 brakes and looking for recommendations re DOT 3 vs 4. I know DOT 4 has a higher boiling point than DOT 3 but is also more hygroscopic, so what am I trading off by using one over the other? Car will probably never be driven hard enough to justify the higher temp capacity of DOT 4, and will probably be inactive for longer periods, esp winters in the South which would expose it to more moisture/condensation.

So I'm leaning towards DOT 3. Suggestions/opinions?

Thanks for your inputs.

[Duke W.]

Re: C1 DOT 3 or DOT 4 brake fluid?

I've never heard that DOT 4 is "more hygroscopic". What is your source for that claim?

I have "heard" that DOT 4 has more corrosion inhibitors, but that was from a vendor, so I take it with a grain of salt.

DOT 3 should work fine for your application. Just make sure you flush the system at a time interval appropriate for your climate, driving conditions, and storage environment.

Duke

[David M.]

Re: C1 DOT 3 or DOT 4 brake fluid?

It may come down to a personal decision between DOT 3, 4 or 5.

I have heard of DOT 4 being more hydrophilic as well. DOT 4 is more prevalent partly due to the lower viscosity which is preferred in today's anti-lock brake systems, and its higher dry & wet boiling points.

Brake fluid or any organic or synthetic based hydraulic fluid is prone to break down due to age thermal cycling and moisture. That is why we SHOULD be bleeding our brakes at min every 5 years regardless of the chemical make up (exception may be DOT5, still cheap insurance to flush).

I went through the entire system on my 68 442. Restored everything master, booster, calipers, cylinders lines et al.
Mike Gibbons from BrakeBoosters.com did a bang up job on my booster and master. He recommended DOT 4 as its much less caustic to paint. The 442 Master which is the same as the Vette master is painted black from the factory, not natural. Thus my decision to go with 4. Plus DOT 3 is old news.

The parts you installed are modern why not do a little upgrade to 4 or better yet 5.
You should investigate DOT 5 full synthetic if everything in your system is new and has never been exposed to DOT 3 or 4.
If I did this all over again Id go with DOT 5 as it is NOT water soluble (100% hydrophobic) and is designed for long term storage because of that.

Here is a decent article without bias:

https://blog.amsoil.com/dot-3-and-dot-4-brake-fluid-whats-the-difference/

[Stephen L.]

Re: C1 DOT 3 or DOT 4 brake fluid?

My vote is Dot 5! Put it in my 67 about 30 years ago, no problems since.

[Rod K.]

Re: C1 DOT 3 or DOT 4 brake fluid?

Sorry for the delay in replying, I was off the grid over the weekend, but thanks for the replies.

Duke: Source of info re hygroscopicity is several articles from a Google search and an Amsoil article on Auto Zone's site. I would not rate all articles as authoritative, but the consensus is that 4 is more hygroscopic than 3 and at least one article (which I can't find now) stated actual values for % of water allowed, etc., as well as wet and dry boiling points. Also, DOT 4 has added lubricants, etc., to allow ABS components to operate properly, which I don't think I'll be concerned with in my application.

Believe I will go with the DOT 3 as I used that as an assy lube when rebuilding the cylinders and don't want to risk the possibility of bad results if I were to use DOT 5.

Thanks again for the responses.