Brake fluid

[Rex T.]

Is there a difference in "synthetic" and "silicone" brake fluid? I remember from years back reading and hearing that silicone fluid should be used in a racing type application. Now about the only thing you see on the shelf is synthetic fluid, Dot 3 and Dot 4.

Thanks,
Rex

[Duke W.]

Re: Brake fluid

ALL brake fluids are "synthetic"!

Both polyglycol ether based DOT 3 and 4 and DOT 5 silicone based brake fluids are "synthsized" in a petrochemical process.

"Synthetic" has just become another word in the marketing hype lexicon.

DOT 3/4 and DOT 5 silicone based brake fluids are not compatible and should not be mixed. Do not "convert" to silicone by flushing. Only way to convert to DOT 5 if you start from scratch with everything in the brake system clean and dry, and begin by assembling ALL hydraulic components with DOT 5 fluid. This will mean disassembling, cleaning with denatured alcohol, and drying any new store-bought components since they are always assembled with DOT 3.

Though more expensive, I prefer DOT 4 over DOT 3. The two are completely compatible since both are polyglycol either based, but DOT 4 has higher wet and dry boiling points and may have a better additive package to help prevent corrosion.

Nevertheless, I recommend flushing brake systems every two years, but if you live in a very dry climate like the southwest, four years is probably okay on an infrequently driven car that is garaged and not driven in the rain.

Duke

[Frank C.]

Re: Brake fluid

Rex...I've had Dot 5 silicone in all of my cars for about 15yrs and have no complaints. I read a comment in Vette Vues from a reader in Colorado that said the altitude and Dot 5 didn't like each other. This is the only complaint I've ever heard....Frank

[Joe L.]

Re: Brake fluid

Frank-----

I've had problems with DOT 5 at high altitude. However, I've also had problems with DOT 3 at high altitude. Actually, I don't think that the problems have anything to do with either fluid.

To settle this, I once consulted with a friend that lived in Albuquerque, NM. He used DOT 5 silicone fluid in his cars with no problem, at all. Since Albuquerque is at about elevation 5,400', I figured that was a pretty good indication that DOT 5 will work well at higher altitudes.

[Duke W.]

Re: Brake fluid

I think DOT 5 has a greater propensity to dissolve/entrap air - don't shake the container. If you install it at sea level and then drive to high altitude the entrained air can expand and cause loss of pedal.

I think this is the source of the "high altitude problem" with DOT 5.

Duke

[Joe L.]

Re: Brake fluid

Duke-----

I used to think that might be the problem, too. However, I've taken samples of both DOT 3 and DOT 5 and placed about 4 oz in clear glass vials. Then, I vigorously shook both to aerate them. Both released all entrained air within just about the same amount of time.

Now, of course, it's very difficult to determine from this sort of experiment if ALL entrained air was actually released. However, I can say that both fluids returned to complete clarity with absolutely no hint of cloudiness. So, if there remained any entrained air, the bubbles were too small to cause even a slight cloudiness. I really doubt that there was any remaining entrained air in either fluid.

[Ray G.]

Re: Brake fluid !

Hello;
Silicone fluid has the advantage of not removing paint or staining plating.
The only complaint is seepage is more common.
Remember this advice is free and worth it.
Ray

[Jack H.]

Good Point, Duke!

Up here in the Colorado mountains, we don't see problems with folks using DOT-5 brake fluid. The problem(s) come when the brake fluid was installed at low altitude, the car was driven/used, and THEN came to a high altitude environment. It seems DOT-5 is areoscopic (affinity to pick up air) vs. DOT-3/4 being hydroscopic (affinity to absorb water).