Dot 5 is Silicone based, Dot 3, 4 and 5.1 are Glycol based fluids. Mixing Dot 5 with any of the others will not only damage the seals it turns the Fluid into a Mush. Any second hand Caliper or Master Cylinder, if it is not known what fluid was being used, should be stripped completely and the seals replaced before use. Dot 5 usually has a Purple colour to the Fluid, the others tend to be Yellow.My 1980 Sportster came new with Dot 5 brake fluid. I was told by the supplying dealer, Oxford Motorcycle Engineers not to mix 5 with other fluids as it could damage seals.
Not sure if Harleys have a totally sealed Brake System but Cars usually have a breather hole in the Master Cylinder Cap where moisture gets into the Brake Fluid.continually being told how Brake fluid is Hydroscopic { like its some kind of weird wizardry or witchcraft}
and that it easily becomes ineffective with water contamination. this may be so , but how is any water going to get into a completely sealed system anyway, its not open to atmosphere !
a warning light just came up on my car saying brake fluid change required, because of either mileage or a period of two years. Why no contaminents or moisture has got in there. B.M.W quoted me £400 to change the fluid . I replied with ive never changed fluid just for the sake of it on any other car ive ever had before, I think its just a main dealer scam whether H.D or any other make
Sealed until you take the cap off to refill or check. I’ve used 5 in some of my old brits just to protect the finishes if leaks happen.There is a hole in the master cylinder lid but there is a membrane gasket ( if that's the correct name) under it so the fluid is sealed from the atmosphere.
I have never understood why Dot 5 Brake Fluid was not used more by Car and Bike manufacturers.