Use of Dot 5 brake fluid

[martym00se72]

Hi All,
Just looking to see if there is anything stopping me from using Dot 5 high temp brake fluid in my 156 TS? At the track yesterday during the Peter Hall memorial 6 hour, towards the end of my 30 min stints, I found that I started to lose pedal as things got hot. I am running Hawk Black pads (and they are awesome on the track) on the front but I think they worked so well and got so hot that the fluid boiled eventually. It is/was fairly new fluid and I have every reason to think it was quality fluid, but I had to clean up the filler cap after the first couple of runs as it had overflowed...

[Citroënbender]

That sounds pretty extreme. Sure there wasn't a little air trapped in the ABS bloc?

[martym00se72]

How do you diagnose that? The pedal was good to start with and stayed that way for the bulk of the time - Just progressively more travel as things got hotter. Should I just bleed the brakes with some Dot 4+ and see what happens?

[Citroënbender]

Glycol ester has a thermal expansion of well under 0.1%, if you take this as a main ingredient in your DOT4, you can work backwards, syringe-measuring the volume change between too full and where it was you started the race.

Say it's 20ml, if you take that as 0.1% of the system volume does it seem likely that you have 20/0.001 ml of fluid in your system? 


Edit for correctness of material specification and nomenclature.

[bazzbazz]

You can't use DOT 5 fluid as it is silicone based and not compatible with your system.

You can get RACING spec DOT 4 fluid with a much higher boiling point.

[Citroënbender]

5.1 is glycol based. 5.0 is silicone fluid.

[martym00se72]

Thanks guys. A fountain of knowledge as usual!

[Colin Edwards]

Motul 660, Martini RF700 or Castrol SRF brake fluid.  All Dot4 and compatible with the seals on your 156.  At more than $110 a bottle, the Castrol SRF isn't cheap!

[GTVeloce]

I'm a fan of Penrite;
https://www.penriteoil.com.au/applications/motocycle/brake-clutch-fluids/racing-brake-fluid#/
It's very reasonably priced so I wouldn't hesitate to replace fluid before every event.
But make sure you aren't mixing silicone fluid (5.0) with non (5.1).

[MD]

I have experienced a crystalline deposition in components from using Penrite racing brake fluid. I raised this matter with the company as being a potential hazard from blockages but they dismissed my feedback. I have no such problems using Motul. Not cheap but it is the best.