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I will be rebuilding the master cylinder in the next couple of months and the question of replacing DOT 3 brake fluid with DOT 5 is bothering me. I have been told that DOT 5 is better because it does not absorb water, but that it is incompatible with DOT 3 fluid..

Recently I have been told that DOT 5 is fine to use so long as I flush all the DOT 3 out of the system. Any opinions out there would be appreciated.

Why not use dot4 and not worry?

oldbmwmc wrote:I will be rebuilding the master cylinder in the next couple of months and the question of replacing DOT 3 brake fluid with DOT 5 is bothering me. I have been told that DOT 5 is better because it does not absorb water, but that it is incompatible with DOT 3 fluid..

Recently I have been told that DOT 5 is fine to use so long as I flush all the DOT 3 out of the system. Any opinions out there would be appreciated.

hi to all, this is going to open a can of worms " BUT " ive been using belray dot 5 since ive rebuilt my R90 around seventeen years ago and was also using it in my other R90 before that , was " TOLD " that it would leak and " NOT" to use it at all , the only thing that if you do use it'll feel a bit spongy over the dot 4 fluid but i can live with that. the main reason i use dot 5 is to save my paintwork " if " the under tank master cylinder leaks it wont (shouldnt) destroy my very expensive paintwork . thats my two cent worth.

I woudn't have thought there was any good reason to be using Dot 3. Can you even still buy it. The latest poly glycol based brake fluid is Dot 5.1 which has higher dry and wet boiling points than Dot 3 or 4.

Dot 5.1 is also thinner. Would that make getting rid of oil bubbles easier ?

So DOT 5 is silicon based and 5.1 is not?

hal wrote:So DOT 5 is silicon based and 5.1 is not?

hi hal, yes that is correct to my knowledge . if your going to try DOT 5 flush the complete system really well that's what i have done in the past .

http://en.wikipedia.org/wiki/DOT_5

Should 5.0 only be used after a full brake system rebuild with new seals 'n all?

Dot 5.1 is poly glycol based like Dot 3 and Dot 4 and you can mix them all safely. What's cconfusing (besides it being designated 5.1) is that it's sometimes marketed as "100 % synthetic" but that does not mean it's silicone based like Dot 5. I have had Dot 5.1 in my airhead for some time now without any problems.

I don't think you can say 5.1 is always better than 4. Perhaps better than run of the mill Dot 4 but there are high quality Dot 4 fluids that have just as high wet boiling points as 5.1. In fact they match all the 5.1 criteriea except for being too thick. One of the reasons the 5.1 spec is thinner is to work better with modern ABS valves. I don't know if 5.1 use in an airhead gains any advantage from being thinner, maybe easier for the bubbles to rise.

ME 109 wrote:Why not use dot4 and not worry?

+1.

SteveD wrote:http://en.wikipedia.org/wiki/DOT_5

Should 5.0 only be used after a full brake system rebuild with new seals 'n all?

hi steve , all i can say on that is i " never " fitted new seals to both of my brake calipers when i rebuilt the bike ( OE seals ? ), and when i bought a " new " OEM under tank master cylnder & braided lines i just bleed the whole sysem really well and filled it up with belray DOT 5 , its only recently that both calipers where rebuilt after one started to weep other than that both calipers bodys where only painted and that was around seventeen years ago as ive said before. just remembered that i use to buy belray DOT 5 off don wilson motorcycles back in the days ( BMW only specialist )