Master cylinder

The Rvalves are right in the outlet ports, behind the tube seats. They are one way valves designed to keep about 8 psi in the lines to keep the wheel cylinder cups in firm contact with the bores. Almost Obviously you wouldn't want to keep 8 psi in the calipers, keeping the pads dragging on the rotors.
Some guys just drill them out. Others pull the tube seats out, and remove the valves, then reinstall the seats. You only get one shot at it tho, so if you wreck the tube seat, you wreck the M/C.