difference in brake fluids

[saltshaker]

what is the difference between dot 3 & dot 5 brake fluid and what should I use?

[Tinker51]

In a nutshell?
Dot 3; Mineral based, will absorb water, will eat paint.
Dot 5; Silicon based, will not absorb water, will not eat paint, higher boiling point than Dot 3.

Dot, 5 does have some drawbacks besides cost. You need to have a specific reason to really need 5 over 3. Just my two cents.

[undy]

Some DOT 3 brake systems will not tolerate DOT 5 (Silicon) fuilds as a substitute. Fuel hose linings, certain gaskets, lip seals and o-rings will deteriorate prematurely when DOT 5 is used. Check to see if your componants and hoses are silicon compatable first.

[Bob In Ct]

Do not use DOT 5 unless you are racing. DOT 5 does not absorb water and thus leads to rust in the plumbing. Use either DOT 3 or 4 and change it now and then.

Bob

[PurpleVenom]

The absorption of water would cause rust. What I learned while installing my electric power brakes.

DOT 3 & 4 are Glycol based. They are used in production cars due to their ability to carry out contaminants within the braking system. These fluids attract moisture, so any moisture that would enter the system would be carried through and flushed out or diluted enough to not matter. Once you open a bottle of DOT 3 or 4 and sit it on the shelf, it starts to become contaminated, and if left long enough, cannot be used. This is why when bleeding your brakes with DOT 3 or 4, the fluid that comes out must be disposed of. BTW - It will eat the powder coating off the frame and will eat through the ceramic coating on your headers, I know this from experience unfortunately.

DOT 5 - silicone based. Does not carry contaminates through your system. Should be flushed through periodically with de-natured alcohol. Suggested by my electric power brake vendor to be used in my system. Higher boiling point was also a reason for me to install since my brake lines are so close to me headers.

Google DOT 3,4, and 5 - there is a wealth of info out there.

HTH,
Tim

[bobcowan]

I agree with all of the above.

Essentially, the higher the DoT number, the higher the dry boiling point. In racing, brake calipers can get really hot, and boil the fluid. Then it becomes a vapor and is compressable. Then you get a soft pedal.

3 and 4 mix just fine. But if you mix 3/4 and 5, it will clot. Then you get real erratic braking. I'v also heard fluidthatixing them makes them more corrosive, but I'm not really sure about that.

There's a new fluid that came out a couple of years ago, DoT 5.1. It's a glycol based fluid, jsut like 3 and 4. But has the performance charectoristics of 5. I have never used, because I have never been able to find it. I use Wilwood.

[patrickt]

Originally Posted by bobcowan

There's a new fluid that came out a couple of years ago, DoT 5.1. ... I have never used, because I have never been able to find it.

Bon appétit. http://www.synlube.com/brake.htm

[Bobcat]

What Undy said X2 . DOT 5 in my Galaxie`s system ( drum rear and disc front ) caused all seals and hose to start to disintegrate . Luckily , I caught it before major damage was done .

[jwd]

Originally Posted by Bobcat

What Undy said X2 . DOT 5 in my Galaxie`s system ( drum rear and disc front ) caused all seals and hose to start to disintegrate . Luckily , I caught it before major damage was done .

Must be a Ford thing. I've used DOT 5 in several vintage Corvettes and have never had a problem. The DOT 5 in my 1960 Corvette has been in there for over 30 years and it looks like you just poured it out of the bottle.
There are many misconceptions about DOT 5 and several of them have been posted here. One of them is that is ruins rubber seals. If it won't hurt a delicate paint job, how does it disolve rubber? It deosn't. Another, mixing it with DOT 3 or 4 will cause it to clot (or whatever). Many years ago, I mixed the 2 in a bottle and agitated it. After a period of time, it did seperate but that was all. I even tried heating the fluid with a torch. After sitting on the shelf for several years with no problems noted, I threw the bottle away.
DOT 5 has been used by many, many car restorers for over 50 years and I've never read of anyone having a problem (except rumors spread on the internet by people who have never actually used it). The military has been using it since the 50's with no problem.

Jim

[WildBill3]

Motorex makes a 5.1 we get it at our local KTM motorcycle dealer never knew it was so good :-)...

[bobcowan]

Since I had some 5 lying aroun, I mixed it with some 3 that I also had lying around; mostly because I didn't have anything better to do at the moment. It was easy to tell the differance, as one was purple and one was clear. It didn't mix well. Not as bad as oil and water. But it definatly clumped/clotted.

5 is also hydrophobic; it will not absorb water. All brake systems will get water in them from the atmosphere. If the fluid is hydrophobic, water will pool. When it gets to the caliper, it will boil and vaporize with moderate braking. That's a bad thing.

3, 4 and 5.1 are hydrophyllic, they absorb water. Since the mixture will be pretty much homogenous, it won't boil as easily. That's a good thing.

As for the seals, 5 won't destroy or eat anything. You can use the standard parts - including rubber parts - with 5. But, the makers of brake parts say that mixing the two will create a caustic substance. I'm not sure how that happens, since they don't mix well. I'v never actually tested this statement. But the people who make brake parts say not to do it, and that's good enough for me.