You loosen the M/C from it's mount, and withdraw it from the mount. This relaxes the pushrod, and allows the internal spring inside the M/C to push the piston back into the parked position, and that opens the C-port, and that allows the fluid to return, unlocking the brakes.
Oh I guess I accidentally deleted this info.I'll transfer this explanation into post 15.
As to what it's for: is to bring in fresh fluid in compensation for wearing brake pads and shoes. Without it, the pedal would continually drop and over time the pedal would be on the floor.
