yes it lets air in, to allow fluid movement in and out of the MC - the level should be slightly below what the inside of the cap would touch, not filled to the rim - also, you may find the system hasn't had the air fully bled out and that air is pushing back against the fluid in the MC .... the exposure to ambient air is the drawback of the old simple systems, the fluid will slowly absorb moisture, so should be flushed and changed occasionally
Bill
