Vents are built into the reservoir caps (brake & clutch). They're pretty small & you usually have to dismantle the cap to find the vent. For the brake its buried in there along with the wiring for the float switch. But I guarantee those caps can't be airtight or you'd be blowing MC shaft seals.

Brake fluid gets pretty corrosive after it absorbs some moisture.

I don't get too anal about it, unless I have to spray water right onto the reservoirs. Besides I flush the fluid every 2 years anyway.