Float inside the brake fluid reservoir gets hung up on it's pivot, too, or held down by a wrinkle in the rubber bellows in the cap if somebody doesn't straighten it up right before reinstall (top up the fluid, then tuck the bellows back up into the cap with all the folds right before you screw it on). It follows the dropping fluid level, isolating the fluid from atmo water vapor which the fluid will suck up like a tick, then when the fluid gets hot enough (like in the caliper), the water boils, and the steam bubbles kill ALL the brake pressure, and you whambo whatever gets in your way, quite hard...