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What happens if you put mineral oil (as used by magura, shimano etc) in a dot 5 system e.g. avid, hope? Any real differences?
I don't want to buy more brake fluid if I don't have to.
The seals swell and the brake fails.
you ruin the system. Hope are dot 4 or 5.1 anyway. This is also incompatable with dot 5
Buy the right stuff. Its cheap.
Dot 3 , Dot4 and Dot 5.1 are glycol based: highly flammable, will damage paint and harmful to the environment if poured down the drain.
Dot 5 is silicone based is not highly flammable, not harmful to painted surfaces (and was developed for the US military for use at extremes of temperatures from Artic to hot deserts) but is still OK to mix with glycol.
http://www.ruderacing.co.uk/Brake_Fluid/c2395/p420600/Automec_Silicone_Brake_Fluid_(DOT-5)/product_info.html
It is possible that people believe that Dot 5 causes rubber seals (etc) to swell because some people unnecessarily use solvent to clean out brake lines between fluid changes.
http://www.britcycle.com/Manuals/DOT5.htm
mccavity - it you mix dot 5 with dot 5.1 sometimes it forms a gell that stops the brakes working IIRC