Getting ready to fill and bleed my brake system. Upgraded the system with a disc brake conversion on the front, new master cyl, and totally rebuilt rear drums. Rubber lines replaced and hard lines removed and cleaned. Any comments on synthetic vs regular DOT 3 fluids.

 

I've never heard of "synthetic" brake fluid.

What's supposed to be the advantage? Is it more expensive?

DOT 3 or DOT 4 should be fine (the difference is the heat range - DOT 4 boils at a slightly higher temperature, making it more resistant to fade in racing-type applications, but gets thick at a slightly warmer temperature than DOT 3, making it unsuitable in extremely cold climates).

I personally don't like DOT 5 (silicone) brake fluid, because it retains microscopic air bubbles and gives a slightly spongy feel.

- Eric

 

Actually Eric this is another case of me not being observant enough at the store. I didn't realize it was synthetic until I got it home. A quick google search didn't shed a lot of light on the subject other than it is OK to use as long as it is DOT 3 or 4.

 

All brake fluid is synthetic. for 99% of street use you will be just fine with DOT 3. In my area many of the street strip guys use DOT 4 because our local drag strip has a very short shut down area and they boil their brakes now and then with DOT 3. (or so they say) I only went down the track one time but my car was not fast enough to worry about the shutdown.

I did see not long ago at Walmart a brake fluid listed as synthetic only. You had to read the fine print to see that it was DOT 3.

 

Yeah, seeing as how conventional brake fluid is not a refined hydrocarbon, but is actually a separate alcohol-soluble substance (glycol-ether), that's what I would have thought, in that the word "synthetic" refers to oils that are not derived from simple cracking alone, but are chemically processed to achieve more uniform molecule sizes and conformations.

So, if it's not a simple "cracked" product, then, by that usage, it HAS to be "synthetic," and since glycol-ethers (an alcohol joined to a short hydrocarbon by an oxygen) are not the products of petroleum cracking, then I guess you can legally call them "synthetic," even though there is no other way that they could possibly be made.

- Eric

 

I used some valvoline brake fluid a while back that said synthetic right on the front.

Also stated it was compatible w dot 3 or 4. I bought it because it's what they had not because of any selling or marketing promo. I have had no issues w it

 

I used the Valvoline synthetic on a couple of my cars several years ago when rebuilding the systems because it was not supposed to attract moisture like standard brake fluid. I was under the impression that it was no longer available and was only on the market for a relatively short time - maybe a year or so. It has always performed fine and is still in the cars. I didn't use Silicone because there were too many negatives making the rounds at that time.

 

Thanks for the replies guys. This was Wearever brand from Advanced Auto. The Synthetic reference was in small print. I filled the system and bled the brakes today. Car may not run yet but it should stop! By the way I purchased a vacuum pump brake bleeder for this. First time I've used this to bleed brakes. Usually I get someone to pump them up for me. Man did that make the job easy and fast with 0 mess. Highly recommended.

 

Over here "synthetic" brake fluid refers to a silicone based product, not compatible with regular dot 3 or 4.
It's fine, probably better than old style fluid inasmuch as it isn't hygroscopic, that is it doesn't absorb water, and has a very high boiling point.
BUT, if you want to use it you must replace every single component in the hydraulic system, which probably rules it out for most of us.


Roger.

 

don't mix silicone with regular brake fluid, the rubber will swell and lock up brakes. We did it a few times, m/c and wheel cylinders locked up

 

"Synthetic" - just another mass-marketing buzzword of the modern ages...

 

They are still selling Valvoline brand dot 3 or 4 synthetic. I just bought some and also had been using it for two years with no problem.

 

I still am somewhat confused by the posts.

So is it ok to use any non-silicone based DOT 3 brake fluid to add brake fluid to a 1950's Olds?

 

Yes, but if the fluid in there is old, you should replace it

- Eric

 

DOT 5 does not destroy paint. If you have a new system then I would use DOT 5.
Note that DOT 5.1 is not silicone based and is not compatible with DOT 5.
I may be lucky but I have never had any spongy feel from my DOT 5. I have read that bleeding with DOT 5 should not be rushed, but I have never had any problems.

 

There are just too many choices for fluids these days. I went to buy gear lube for my rear end last week. There was 75w90, 80w95, 90w140, and about a dozen other blends. I got to the counter and it rang up for $12.75 I couldn't believe it. The guy at the counter pointed out that it was synthetic which I had failed to notice. So now I go back to the rack and find out I have a choice of all these weights in synthetic or mineral. Holy crap! I just wanted a jug of 90 weight.

 

One thing not mentioned so far. When we did my'59 It has a hydraulic brake light switch. Silicone brake fluid attacks the internals and will cause it to fail. I was replacing them every two years until I found Harley Davidson offers a exact looking one. The only difference is the threads so a adaptor had to be used. That was five years ago.

 

I hadn't thought of that, thanks for pointing it out.
I shall tell a buddy who is restoring a Morris Minor and was thinking of using silicone as he was replacing all the brake components.
You may have save another old car enthusiast from trouble later on.


Roger.

 

Dot 5 silicone fluid is very spongy felling, Harley used that stuff for several years, I had a couple of bikes that used it, it worked OK but is spongy.