: DOT 3 in a DOT 5 brake system

10-10-2008, 12:47 AM
A friend is nervous because he accidentally put dot 3 in his brake system.....it was new aftermarket stuff and never had dot 5 in it. But after cleaning up he found the de-structions that said "use dot 5"

What are the effects if any? again....it was all new stuff and never had any other fluid in it.

10-10-2008, 12:52 AM
the big difference is dot 5 is silicone based and has a different boiling rate... here read this
and make sure you dont mix the dot 3 (or 4 for that matter) with the dot 5 , flush the system...


10-10-2008, 12:56 AM
Thanks....I knew the difference, but he is under the impression that his dot 3 fluid is going to cause some kind of failure in the seals or something and I say bs.

10-10-2008, 01:21 AM
Never say BS unless you KNOW it to be BS. And no I don't know the difference. But I do know they didn't call it dot 5 because it was the same as dot 3.

10-10-2008, 02:55 AM
i was told not to mix them. apparently the systems are the same though, you can run dot 3 or 5 in the same setup, just not together. but i aint no expert no how

10-10-2008, 06:51 AM
Rite. You can not mix.
Dot 5's main use is applications that need a higher boiling point.

On a bike, if you have brake lines running close to exhuast. The fluid can boil in the line, & you have instant no brakes.

Road race cars that heat the calipers up to insane temps.

Your question, no putting the Dot 3 in shouldnt have hurt anything.

At least on a bike to easy to flush & bleed it through.

Larry T
10-10-2008, 07:42 AM
I use DOT 5 in my bikes because it doesn't eat the paint when it spills on it. I always thought most race cars stayed away from DOT 5 fluid. Something about water won't mix with DOT 5 and condensation causes water pockets that turns to steam causing the brakes to go away. Also heard that DOT 5 gets spongy at altitudes, too.

If you mix DOT 3 and DOT 5, they turn to jelly. So if you change 3 to 5 or viceversa, be sure to flush the system real well.

I'll bet the DOT 3 will work in place of the DOT 5, but if there is any doubt, just pick up the phone and call the manufacturer.
Off for coffee,
Larry T

Ivan RoachCoach
10-10-2008, 01:21 PM
The Dot-3 will absolutely fuck the seals in a DOT-5 system. But not vice-versa.
Have your friend drain the fluid and thoroughly flush his system with denatured alcohol from the hardware store.

I converted my DOT-3 setup to DOT-5 not long ago and this is the way to remove any residual traces of the old DOT-3 from your mater cylinder, calipers, and brake lines if you don't have the coin for all new components (who does?).

Don't worry about what you've heard that race car guys do or don't do.
It's your friend's ass on the line, not theirs.

10-10-2008, 02:42 PM
The Dot-3 will absolutely fuck the seals in a DOT-5 system.

Thank you....thats the answer I was looking for.

10-10-2008, 04:28 PM
they dont mix. it turns to jelly almost immediately.

some natural rubber seals will swell if silicone dot 5 is used. brembo, isr, to name a few are adament not to use the wrong fluid, glycol based only according to them because they use natural rubber. isr marks every box, dot 4 only.

evidently with other/most synthetic seals and orings it doesnt matter either way, but you still have that jelly thing happening if you mix them, and it seems to harden up over time and turn crusty..

i usually repair a few of these screwups a year.