Red'n'Black
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- 536
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- Malvern, Worcs
On motorbikes, yes, presumably so the fluid doesn't all leak out if/when you drop it. Never seen one on a car M/C, the cap usually has a tiny vent instead.Theoretically the brake fluid should never have any contact with the atmosphere, once it’s in and the cap is on. There’s a flexible rubber seal on top of it that rises and falls with the level, as pads wear.
This notion people have of topping up the brake fluid when it looks a little low (because the pads have worn) is probably what causes the problems with contamination.
And also what causes problems when they try to fit new pads with the master cylinder cap still on….