Brake fluid

[kerryg]

FWIW I thought I'd share this.

Last weekend I went to Motomail to get some brake fluid. I wanted DOT3 or 4 (cos that's what the manual says) and they didn't have any. The young man who served me said (I'm paraphrasing him but the gist of it was) "we don't stock DOT 3 or 4 cos this [pointing to bottles marked DOT 5 and DOT5.1] supersedes it". I was not sure so decided not to buy, having in my mind some nagging worry about some brake fluids not being compatible.

I did some research and it seems the young guy was right...PARTLY. The DOT5.1 is glycol based chemistry that can substitute for DOT3 and 4 (also glycol based), no problems, BUT (it's a biggish but) DOT 5 is different. It's silicone chemistry and is not compatible with/miscible with the glycol based fluids.I don't know what risks (if any) might result from mixing the two but it seems a bad idea to me (like oil and water, they stay separate).

[curious george]

There are some DOT5 fluids that are designed for racing only, (the silicon ones), and glycol based ones which are compatable with the older fluids.

No, you cannot mix the two at all, you need to clean out the whole calipers, hoses and master cylinder if you use the silicone one.
I don't think many racers use it because it is so hydroscopic either... it's a pain changing it for every race.

[Mr Skid]

IIRC DOT 3 / 4 / 5.1 are all from the same family and DOT 5 is yuccky stuff not for garden variety applications.

I think that 5.1 is less compressible than 3 or 4 but is will absorb moisture more quicky and need changing more often.

I'd don't think that you couldn't go wrong with OEM advice though.

I've delt with Hi Tech Brake and Clutch ( 1 MELROSE ST
NEWMARKET) before and found their service good, the other big place I'd suggest would be 0800brakes (351 New North Road, Eden Terrace)

I'd expect they'd carry decent quality DOT 3 / 4 fluid in stock

[Posh Tourer :P]

Apparently DOT5 is incompatible with some seals in DOT3/DOT4 systems. Do not mix. You might end up wearing out the seals and blowing them.....