If this occurs, spray immediately with brake cleaner to remove completely. They must be replaced at the same time as the pads. There isn't sufficient metal thickness to safely accomplish this. It helps when you need to check if your warranty is still in effect. More importantly, it's a great gauge of performance and an indicator of other problems.
Most specialists recommend changing the fluid with every pad replacement.
If you're unsure of the proper technique for bleeding the hydraulic system, don't perform the job by yourself. Seek help from a professional. ABS equipped cars should be bled only by professionals. However, whenever replacing pads, you should also replace the sensors and seriously consider replacing the rotors at the same time. This conditions them for maximum performance and prevents premature failure.