[thstone]

The fluid in the reservoir is used by the braking system to "add" fluid as the brake pads wear (they get thinner). This maintains fluid in the system at all times.

When you replace the worn (thinner) brake pads, you'll need to push the caliper piston back into the caliper so you can get the new (thick) pads to slide into place between the rotor and the piston. When you push the piston back into the caliper, brake fluid will be moved back into the reservoir.

There is no need to replace the fluid in the caliper - it is continually maintained "full" by the reservoir.