Front Brake doesn't feel right

check with the dealer.. for a service bulletin on the MC..

I've changed to Motul DOT 5.1. The original Fluid was black after 1500km and I had suddenly fading brakes on the track. 5.1 is still clear after more than 2'000 km on the track. No fading.

rocroc ch said:

I've changed to Motul DOT 5.1. The original Fluid was black after 1500km and I had suddenly fading brakes on the track. 5.1 is still clear after more than 2'000 km on the track. No fading.

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Thanks! I didn't know that 5.1 exists! I only was aware that you should never pour DOT 5 into DOT 4 Systems.
I used Castrol SRF, but it became simply too expensive. I guess DOT 5.1 could be the solution.

The dry and the wet boiling point of DOT 5.1 is about 30 till 40° Celsius higher than DOT 4. Only racing fluids have higher boiling points but absorb more moisture than a "normal" DOT 5.1. Professional racer will use racing fluids and change them after each race. I change the DOT 5.1 once a year.


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You should scratch DOT 5 from that list. DOT 5 is silicone based and should not be used lines that have ever had DOT 4 or 5.1 in them.

DOT 4 is (ok) for bikes without ABS
DOT 5.1 is for bikes with ABS

There is more to brake fluid than dry and wet boiling points

Motul RBF 600 is has relative high dry/wet boiling points (315°C/205°C) but absorbs water faster than plain DOT 5.1
Motul RBF 660 has even higher boiling points (325°C/205°C) but absorbs water even faster.
Castrol SRF has a bit lower dry boiling point(310°C), but a much higher wet boiling point (270°C) compared to Motul RBF. On top of that it also absorbs water much slower than the Motul RBF. So it has the ideal combination, hence the high price (4x the price of Motul RBF 600/660).

So if you'r OK with changing your fluid often (multiple times a year for track use) go Motul, if you prefer once a year, go Castrol. That is if you brake hard and long enough to experience issues using regular DOT 5.1.

Just my .02 .

PS: note that Motul RBF and Castrol SRF are DOT 4. I have no idea what this means for racing bikes with ABS.