Check the level in the resevior, if it is near the minimum mark it won't overflow, if its nearer the maximum mark yes slacken the cap off and put some rags around it to catch the spillage, also when you've changed the brakes, and pumped the brake pedal a few times, check the level, if it is above the max mark, take the cap off and use a clean dry rag to soak some fluid out of the resevior, to get it down to the max mark.