Unless you have a leak somewhere, you prolly won't need to add any brake fluid, but it may be low. I would NOT remove the master cylinder lid when pressing the pistons back in...unless you happen to like the way paint looks after being eaten by brake fluid.
The best thing is to put a hose on the bleeder, and run the other end of the hose into a can/bottle of clean brake fluid. Open the bleeder slightly, and push the pistons back in the caliper, then close the bleeder. This will keep from pushing dirty fluid back thru the lines, up into the master, and it will also keep the master from overflowing.
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Joel Adams
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"