brake fluids, dot 3, dot 4, dot 5, wtf?

[240meowth]

so yah, i'm planning changing my brake fluid, but i don't get which one to get, i heard dot 4 is a compromise. but what brand, and where can i get 'em for good price?

[adey]

Dot 4 a compromise: because it has a higher WET boiling point (which is what DOT bases its ratings on), the DRY boiling point may not be as high as some quality DOT3 fluids. This is the only compromise. If you're getting some quality Dot4 fluid, you can probably get away with once every 2 years if your pads last that long... because the WET (i.e. old... been in your car a long time...) boiling point is still pretty high.

Make sure though, that you don't get DOT5, which is race-only and that will eat away at ... uhhh ... stuff in your brake system. I think it eats at rubber? ... or something important like that.

If you plan on changing fluid often (which anyone who drives their car hard... not drag... should), you can go ahead and look for a dot 3 fluid with a high DRY boiling point and disregard the WET boiling point since it won't be in your car long enough to really soak up much water-- which affects the boiling point of the fluid. Did that make sense? There was a good article on this somewhere online, but I can't remember where for the time being. I'll post it if I remember/find it.

edit: you can buy the stuff anywhere, really. Any autozone will have at least 2 different fluids to choose from (I guess that isn't that much choice, eh?) ... I use the valvoline DOT4 stuff because it was the only "good" looking (read: branded) stuff they carried, and had a nice dry (and wet) boiling point of well above 400F. I think it was near 460 or so dry, and 420 or so wet. Don't quote me on that though, I'm running on old memory.

[mrmephistopheles]

http://www.vtr.org/maintain/brake-fluids.html

I posted this in my brake FAQ, very informative.

[240 2NR]

DOT 5 should be avoided, but not due to corrosion. It's actually the opposite. It's silicone based and is compressible (the reason you don't want it, spongy pedal feel), but also far less corrosive than glycol based fluid that can strip paint. DOT5 is actually recomended for classic cars and ones with delicate linings that are driven infrequently.

DOT4 is all you should really need in a street car. Lots of people recomend castrol GTLMA and i liked the valvoline synpower I was running. If you plan to race the car, something a bit better may be worth a look (DOT5.1 naybe), like ATE superblue. Either way, brake fluid isn't too expensive. The first two listed are around $5 per liter and superblue around $10-12.

There are some good brake replacement/ fluid change procedures in the FAQ section. In fact I wrote one.

[kandyflip445]

DOT 5 shouldn't be used in a DOT 3 system. It will cause the seals to swell and eventually give and then you won't have any fluid at all. DOT 4 would be a good choice because of what adey said or just a good DOT 3.

[Aubrey]

Just a side note:
Dot 3 and Dot 4 can be mixed. To run Dot 5 you have to drain all your brake lines and then re-fill.

[kandyflip445]

Originally posted by Aubrey
Just a side note:
Dot 3 and Dot 4 can be mixed. To run Dot 5 you have to drain all your brake lines and then re-fill.

Even if you drain your system and fill it with DOTf 5 it will ruin it.

[ruf]

Switching to DOT5 you have to flush your system with denatured alcohol. Pain in the ass. DOT5 is fully synthetic and does not corrode paint. It comes stock on Harleys because they expect their bikes to leak and they don't want to chew up fancy $10K paintjobs. You don't know HOW many Harley riders I've heard brag about this until I explain to them the reasoning. The reason you don't use DOT5 in high performance applications is that it is NOT hygroscopic. I does not absorb water, so the water ends up accumulating at the lowest point in the system (water is heavier than brake fluid), which is usually the caliper. Now instead of a 400C boiling point, you have a 100C boiling point. Bad things happen when your brake system boils. Most amatuer racers that I know won't even run DOT5 because the bleed/maintenance interval is too high.

IMO, nothing beats a good racing DOT4. Well maybe a DOT5.1 (also DOT3/4 compatible). I personally use Motul RBF600. Nice stuff.

[kandyflip445]

DOT 5 FLUID

DOT 5 silicone fluid does not absorb water and has a very high boiling point. It is noncorrosive to hydraulic system components, and it does not damage paint as does polyglycol fluid(DOT 3,4 & 5.1). DOT 5 fluid has other characteristics that are not so beneficial.

DOT 5 silicone fluid has a lower specific gravity than polyglycol fluid. If the two types are mixed, they do not blend; the silicone fluid separates and floats on top of the polyglycol fluid. There fore, if you want silicone fluid in your vehicle, all the polyglycol fluid must be completely flushed out. The best time to convert to silicone fluid is during a complete brake system overhaul.

Silicone fluid compresses slightly under pressure, which can cause a slightly spongy brake pedal feel. Silicone fluid also attracts and retains air more than polyglycol fluid does, which makes brake pleeding harder; it tends to out gas lightly just below its boiling point; and it tends to aerate from prolonged vobration. DOT 5 fluid has other problems with seal wear and water accumulation and separation in the system. All of these factors mean that DOT 5 silicone fluid should NEVER BE USED in any car with ABS.

The best thing to do is use a high-quality brand of brake fluid of the DOT type(glycol based DOT 3, DOT4, or DOT 5.1) specified for a particular vehicle. Avoid mixing fluids.

As stated before DOT 5 and DOT 5.1 are different. DOT 5 is silicone based and DOT 5.1 is glycol based.

Also on the glycol fluids. Try to compare their boiling point properties. Some DOT 3 fluids will meet all the specs for DOT 3 but have a higher boiling point of DOT 4 requirments. Like I said just look for a high quality fluid.