 You've got to examine every single brake line, every single brake part, examine it with a fine-tooth comb to see if you can find any evidence of a leak. If not, it could be leaking into the brake booster, which the master cylinder is attached to, and that could be where your brake fluid is going. So those areas have to be checked. And sometimes the only way to check a brake booster for a leak is to remove your master cylinder and check it.