DOT 3 vs: DOT 4

[Guest jerroldt]

We went for a 250 mile run last Sunday and I got behind an SUV in the twisties whose brakes were smoking big time. I later found a fine spray of clear oil mist on the frontal areas of the bike. I assume it may have been brake fluid from a leak on the SUV. It did no harm to the paint on the Le Mans. On my last Guzzi, I changed from DOT 3 to DOT 5 for the benefits of silicone etc. My 03' specifies DOT 4. Is this harmful to paint and what are the differences between DOT 4 and DOT 5?

[al_roethlisberger]

Not sure off-hand, although I'm sure a quick Google would turn up a quick explanation...

 

But I used the synthetic fluid from Valvoline, that mixes with either a DOT 3 or 4 system(but is rated as neither), and is rated for very high temperatures, and is resistant to hydrophilia.

 

http://www.valvoline.com/pages/products/pr....asp?product=51

 

I have no idea how good it is, but it seems to work fine

 

al

[emry]

Well assuming that the SUV was using DOT3 or 4 (currently all major manufacturers use one of those, except HD, at least for all vehicles imported to the US, any comment from those in other places???) and it is harmful to your paint. Glad to hear you wiped it off in time.

 

The numbers are used to ID different Department Of Transportaion specifications. The main one being the Boiling point. Dot 3 and Dot 4 are both a gycol based fluid, Dot 4 having a higher boiling point.

 

Dot 5 is a silconce based fluid and is NOT COMPATABLE with gycol based fluids nor should it be used with ABS systems. While it has a higher boiling point then 3/4 it has very poor aeration qualities and is hygrophobic. If water is introduced to the system it merely creates pockets of moisture which can cause rusting etc, not to mention boiling under heavy use.

 

If you are thinking about converting you MUST completely diassembly the entire brake system. Dot 3/4 fluid shrinks rubber, additives are added to help prevent this, Dot 5 causes seals to swell (this was a design requirement, so that it would lessen the chance of fluid leaks over time) which also has additives to help reduce this. If the two are mixed, the seal turn to mush, and under high pressure normally fail. People who do convernt must first soak the entire system in Denautured Alcohol to remove all traces of fluid. Normally people who do this have little or no problems.

 

Then there is DOT 5.1. More confusinion. Ok it is back to gycol based and is compatable with 3/4. But not 5. Once again higher boiling.

[jrt]

Thanks for the info Emry. One clarification- it's probably "hydrophobic" .

J

[docc]

I had used 5.1 on an older disc system with good results . I understand it is 'thinner' for use with ABS systems. I wonder if it would be a good choice for our finicky rear brakes allowing the piston to retract more reliably?

[emry]

You are quite right jrt, good eye.

[Guest jerroldt]

Thanks for all the input. It has been enlightening. I will stick to the mfrs. recommendations for brake fluid. The oil washed off and the bike has a new coat of wax.

[Lex]

Having been the victim of DOT 5 (water in the caliper, rust, locked brake) I'll second the note of caution. This was a while ago, maybe the new stuff is better but I use DOT 4 and change it when, as a very good mechanic put it, "it doesn't look like water". Brake fluid is cheap, easy to change* and easy to recycle.

 

Lex

 

* Unless you have a BMW with ABS II, then it is not so easy to change.