Synthetic brake fluid

[Michael G.]

Is the newer DOT 3-4 ''synthetic'' brake fluid compatible with the older glycol material? It's time to flush (swap-out) and all I see is the synthetic product. I'm not quite ready for the complete transfer to silicone yet. '71 w/ power brakes. Thanks.

[Mark E.]

Re: Synthetic brake fluid

Are you saying you can't find any DOT 3 fluid? I just rebuilt my brake system using DOT 3- it works fine.

[Michael G.]

Re: Synthetic brake fluid

I had started at the Advance Auto Parts (my local supplier) web site and put in "DOT 3 BRAKE FLUID". All listings were for DOT 3 SYNTHETIC. admittedly, I have not actually checked the shelves of any store as of yet. Just noticed that the listings were all for a synthetic product.

[Mark E.]

Re: Synthetic brake fluid

Let us know what you find. I bought my DOT 3 over a year ago- maybe the market has changed.

[Brian M.]

Re: Synthetic brake fluid

I thought ALL DOT 3 was synthetic and always has been??? I have seen some labeled DOT 3 & 4 on the same label. I use Castrol LM DOT 4 .

[Mark E.]

Re: Synthetic brake fluid

I misread Michael's post- I thought he was asking about DOT 5 silicone fluid, but he was asking about DOT 4 synthetic fluid. My bad.

According to Dr. Wikipedia-

DOT 4, like DOT 3 and DOT 5.1, is a polyglycol ether-based fluid (contrasted with DOT 5 which is silicone-based). Fluids such as DOT 3, 4, and 5.1 are hygroscopic and will absorb water from the atmosphere... While a vehicle that uses DOT 3 may also use DOT 4 or 5.1 if the elastomers in the system accept the borate compounds that raise the boiling point (a temperature upgrade), a vehicle that requires DOT 4 might boil the brake fluid if a DOT 3 (a temperature downgrade,) is used. Additionally, these polyglycol ether based fluids cannot be mixed with DOT 5.0, which is silicone based. As of 2006, most cars produced in the U.S. use DOT 4 brake fluid.

So DOT 3 is the stuff that's been around for awhile; DOT 4 is newer, has a higher boiling point and is compatible with DOT 3; DOT 5 is silicone based and not compatible with DOT 3/4.

What's confusing is DOT 5.1, which seems similar to DOT 4 and isn't compatible with DOT 5.

I'm not sure about the synthetic distinction. Maybe some DOT 3/4 fluids are synthetic, some are not? I assume any DOT 3/4 fluid is compatible with other DOT 3/4, regardless of being synthetic or not? Just like some engine oils are synthetic, some are not- but they are compatible.

[Michael W.]

Re: Synthetic brake fluid

All brake fluid is 'synthetic'. No idea where the concept of thinking that only DOT 5 is such came from.

[Pat H.]

Re: Synthetic brake fluid

GM dealers still have DOT 3. I just bought some about a month ago

[Duke W.]

Re: Synthetic brake fluid

All brake fluid is "synthetic" because it is "synthesized" via a petrochemical process using petroleum-derived feedstocks. By the same definition ethylene glycol antifreeze is synthetic, too. So is weed killer and bug spray. Virtually any chemical that is not derived from crude oil by conventional distillation and catalytic cracking can be characterized as "synthetic". The word has become just another marketing hype term.

DOT 3 and 4 are fully compatable and both are usually available at any auto parts store. DOT 4 has a higher wet boiling point and usually a higher dry boiling point, too. Also DOT 4 may have better corrosion prevention additives. I use DOT 4 for my cars that have not been converted to DOT 5.

Duke

[Mark E.]

Re: Synthetic brake fluid

Thanks Duke.