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Dot 3 (and Dot 4, for that matter) are hygroscopic, i.e. they absorb moisture over time. Dot 5, being silicone-based, doesn't. The reason you shouldn't mix them is that they have different boiling points, and as anyone with experience of metals knows, sometimes when you mix two things with different boiling/melting points, sometimes you get a mix which has a boiling/melting point lower than either of its original constituents.
 

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Mixing the brake fluid is like mixing oil. Won't stop it from working but the two shouldn't be forced together because the blends ARE different. I suggest mail order if you can't find it. Anyway, all modern bikes have been carrying Dot 5 fluid since 1987. My K100RS uses it as required by all Euro bikes as of 1995. You can also try braided lines to firm up the brakes. Either way you will improve braking- slightly.
 

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"Also, when regulating the DOT 5 the officials knew perfectly well that it is just a matter of time before somebody in Boise, Idaho, will mess things up. Probably somebody named Wayne."



Well, I guess we probably have our share of uninformed people here in southern Idaho, but not so many that Boise should be held up as an icon of ignorance. My perception is that most of those ignoramuses have showed up here lately, and possibly many of them from wherever cruiz-euro hails from.
 

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I think the author of the above did not fully research the facts about the fluid. Point in question - quoted as follows



Basically it boils down to two simple scientific facts:



1) Anyone who says you can´t mix DOT 3 and DOT 5 is a complete freeking idiot.

2) Anyone who does not immediately change the brake fluid of all his keeper bikes to DOT 5 is a complete freeking idiot.



The above is Not a Scientific Fact. Not surprising coming from a Harley guy...
 

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You obviously did not read the article and thus are spreading the misinformation yourself.



Firstly, you can mix DOT 4 and DOT 5. It was tested with fifty vehicles. Anybody who changes the fluid type does that mixing also, because 10 - 30% of the old type remains in the system. But as stated in the article, no problem.



Secondly, glycol based DOT 4 collects moisture, which causes corrosion in the long run. DOT 5 does not and prevents all corrosion. This is the reason for DOT 5.



- cruiz-euro

 

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Wayne, many came to Idaho where I am from, but that was very long time ago. Those newcomers could be perhaps from Mexico? Finland and Mexico are very similar, we both have plenty of snow apart from Mexico.



- cruiz-euro

 

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They are scientific facts. As Gandhi said: "Screw the freeking idiots". Lemme ask you a question. If you have brake fluid that corrodes the brake system from the inside in a long run, in a bike that you want to keep for many years, what does that make you?



- cruiz-euro

 

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I've never heard that DOT 4 fluid corrodes the insides, but have heard DOT 5 does. Yes DOT 5 does have a higher boiling point, but I doubt you'll ever reach those temps on a Cruiser unless your brakes are extremely under par or you decide to take it to the track (no, not the dragstip). Going from stop light to stop light will not over heat your brakes or break down the fluid immediately(over time, yes) DOT 4 & 5.1 are safe for modern brake systems, although I'm not sure about Harleys since the technology is older (except for the V-rod maybe). If Harleys are made to use DOT 5, then put it in. I personally wouldn't mix the 3&5 for the same reasons stated by another member, lowering the boiling point. I wouldn't use DOT 5 by itself in my bike after hearing it corrodes. Do what you want to do to your bikes.
 

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Don't mix them, and don't use silicon

Statement: Polyglycol brake fluids corrode the braking system

Polyglycol brake fluids (like DOT 3 and 4) are hygroscopic. When they absorb water they tend to destroy the rubber bits in the brake system. Solution: Bleed once a year. Empty the master, refill with clean fluid, and gravity bleed each caliper until you get nice, clean fluid out. I bleed mine at least once every six months on all my vehicles. I have a 1996 Chevy Astro with 94,000 miles on it. I changed out all the brake parts, including flex hoses, for an extended road trip in 1994. Disassembly showed that only the wheel cylinders looked a bit sad. Other than that I had replaced no parts but shoes and pads.

Statement: Silicon fluids corrode the brake system.

Since silicon based fluids do not absorb water, condensed water can pool in droplets and corrode aluminum and non-stainless parts of the braking system.

"Mixing" isn't wise since they really don't mix. Sort of like oil and water, you can get them together but neither will be happy.
 
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