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What do they mean and how is this done.?
Not sure if I have Dot 5.1 or Dot 4 in at the monment.
depends on the brake as to the method but effectively it's pumping new fluid through until all the old stuff has gone rather than just 'topping it up a bit'
You can mix 4 and 5.1 so you don't need to flush, but it is a good idea to flush it every now and then as fluid is cheap. Just keep pumping it through until the stuff coming out is the same colour as the stuff you are putting in. If the fluid has been in there a while it's generally pretty obvious when the old stuff has gone. You need to keep topping up reservoir or reverse bleed from a big bottle to avoid pumping air in.