I recently replaced a rotor and rebuilt a brake caliper because one the brake cylinders was stuck and was gouging into the rotor. I carefully re-assembled everything using brake grease and brake fluid (DOT3) as lubricants were necessary. During the bleeding process a small number of bubbles float up into the servo end of the reservoir. There are no visible leaks in the braking system (from pedal to caliper). The back brakes work fine. I can't get the front brakes bled properly. Is the problem in the master cylinder? Maybe some corrosion? If so, where is the fluid going that is being replaced with air? I see no leaks anywhere. The servo has less than 2k miles on it.