Brake/Clutch Fluids?

Questions run around in my head from time to time. Sometimes they pop out to the big wide world, like this one.

My bike uses DOT4, as per the manufacturer's specification and many of your bikes, if not all. I have heard that DOT 5 (as distinct from DOT 5.1) can be used as a replacement for DOT 4. However as DOT 5 is silicone based (and less hydroscopic - doesn't readily absorb moisture) and the other DOTs are based in the ester world, is it possible to replace DOT 4 with DOT 5. If this is indeed possible, what would you need to do, preparation-wise for this to occur.

As we know DOT 3, 4, 5.1 all will chew out your paintwork, if spilt. From my reading, DOT 5, possibly, will not. As DOT 5 is often used for very low use vehicles or vehicles in storage, why not every day usage?

Thoughts from the experts?

I thought it was the other way round Tony? 3, 4 and 5.1 are all interchangeable but 5 is the odd man out?

I may be wrong though as I only use 4 and change it often, especially in the clutch system.

I thought that is what I said Pete. Now I am doubting myself but that's normal.

I do know that 5.0 was used at work as a Siltherm replacement - apparently the colour of 5.0 is different to the original Siltherm. However buying Siltherm is way expensive when it comes in 44 gallon drums and we needed a few litres only. Notwithstanding all that, it was used in a heat exchanger for something that gets quite hot and sits at high voltage (up to 100 kV) so it's a good insulator too.

I am thinking that if the old system was thoroughly cleaned, then 5.0 could be used instead.

This article is good
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Basically Dot 5 is the silicone based fluid usually purple sometimes called 'synthetic brake fluid' - originally introduced for its higher boiling point and not compatible with the glycol-ether based DOT 3,4, 5.1.

It does not absorb water from the atmosphere like other DOT brake fluids, which gives it a very long life-span but it means that any moisture that remains or gets into the system will boil at 100 deg C (rather than the 140-190 deg C of DOT 3,4 & 5.1 when contaminated with water).

Article also claims that DOT 5 is more compressible than the other DOT brake fluids which can degrade brake lever (or pedal) feel.

Thanks Jon, wonderful stuff

Some of the soft bits (seals and such) don't play nice with DOT5. Use what was intended for the application, change as needed.

rick pope wrote:

Some of the soft bits (seals and such) don't play nice with DOT5.  Use what was intended for the application, change as needed.

Rick, good to know. I might park my thoughts for a while.