Brake Fluid?

BIKER BOB · Mar 13, 2017

Hi folks,
I guess this is the right place for this question. I am working on my 69 Harley and am just about to change master cyl and wheel cyl. The question is: The master has printed on the cap to use Dot 5 only, will it hurt the seals and cups to use Dot 3, or 4 instead? Both parts are brand new and I would rather use something "standard" instead of synthetic. Any opinions or ideas?
I figured someone here would know even though it's not tractor related.
Thanks

Rosher18 · Mar 13, 2017

My understanding is that DOT5 is NOT compatible with DOT3/4 because they're chemically different. DOT5 is silicone-based, and DOT3/4 are glycol-based.

BIKER BOB · Mar 13, 2017

Yup, I meant silicone not synthetic. This system is new and clean right now and the master cyl says on the cap Dot 5, just wondering if Dot 4 will be gentle enough to the seals inside. Thanks

Rosher18 · Mar 13, 2017

I wouldn't put DOT4 in a DOT5 system simply because I'm not sure if the seals would survive the glycol based fluid.

mjw357 · Mar 13, 2017

Also consider, that if you put DOT 4 into a reservoir that is marked DOT 5, a mechanic might top it off with DOT 5 and then you'll have a real mess.

TemporaryKubotaOwnerMark · Mar 13, 2017

DOT 4 is NOT compatible with DOT 5. The numbers do not indicate a grade, but a TYPE. Don't mix them unless you want to rebuild every bit of your brake system.

Just to make things even more confusing, the industry STUPIDLY decided to allow something called 5.1, which IS compatible with DOT4 and is NOT compatible with DOT5!!!!

I think some people really ought to be publicly flogged for such stupidity.

Read more here:

Since DOT 4 and 5.1 are both glycol-based brake fluids they are compatible with each other, which means they can be readily mixed without harming your brake system. It is important never to mistake DOT 5.1 (glycol-based) with DOT 5 which is silicone-based and should never be mixed with any other DOT fluid.

is dot4 compatible with dot5 at DuckDuckGo

BIKER BOB · Mar 13, 2017

Are there any drawback to using Dot 5, other than the fact that I don't have any yet? Its an old bike and Dot 5 wasn't around when it was built. Ordered a new master cyl for it a while back, only to find out when I finally took it out of the package that it is for Dot 5 only. Will Dot 5 hurt the new old style wheel cyl that I just got? I can only imagine that it was made for the old style brake fluid as it was made for a 1965 Rambler (also fits 1969 Harley). Everything is still clean as I have not yet put the system back together, so I can go either route. Just seems that the glycol based fluids are pretty rough on things.

TCJatko · Mar 13, 2017

BIKER BOB said:
Are there any drawback to using Dot 5, other than the fact that I don't have any yet? Its an old bike and Dot 5 wasn't around when it was built. Ordered a new master cyl for it a while back, only to find out when I finally took it out of the package that it is for Dot 5 only. Will Dot 5 hurt the new old style wheel cyl that I just got? I can only imagine that it was made for the old style brake fluid as it was made for a 1965 Rambler (also fits 1969 Harley). Everything is still clean as I have not yet put the system back together, so I can go either route. Just seems that the glycol based fluids are pretty rough on things.

It's my understanding that Dot 5 (silicone) brake fluid is safe for most all systems. Decades ago when I was in the USAF we converted equipment from Dot 4 to Dot 5 fluids; the only requirement was a thorough cleaning with methyl alcohol before the change.

TemporaryKubotaOwnerMark · Mar 13, 2017

It's all one, or all the other. You cannot mix them. If you are converting, you must flush the entire system thoroughly.

I understand the military has decided to go back to the glycol based fluids. Although it is hydroscopic (absorbs water) and that theoretically leads to rust in the system, what happens in practice is that the silicone stuff, while it will not absorb water, will also not prevent it from getting into the system, and if it does, it just sits in low spots and happily rusts away. They found that regular maintenance of the glycol types gave them more reliability and longer life.

There are lots of folks who like to debate it back and forth, but the one thing that is not debatable is that you cannot mix them.

vsteel · Mar 14, 2017

I have had bikes with both types of systems. DOT 5 was supposed to be better in bikes because it can take higher temps but in the real world you are not going to see a difference. 5.1 also threw that out the window.