I really don't think you should tell this to prospective employers. If you are going to do it during your breaks and lunch then it's none of their business.
If you tell this during the interview, they might see you as someone who could be potentially difficult, even if you tell them that you would be "willing to take time out of your breaks and lunches," and actually, ESPECIALLY if you told them that.
As your employer, they will EXPECT you to take care of your personal business during your personal time (breaks, lunch), NOT during the time they are paying you for. Therefore, even using the phrase, "I would be willing to..." might set up a red flag, because the employer's expectation is that those times are the ONLY times that you would be pumping. (How would you be breastfeeding, unless you are working at a day care where your child is present?)
Pumping/nursing your child are great things, but you will be hard put to find an employer in this country who is going to feel like it is their responsibility to pay you for the time you are engaged in doing it. Therefore, I would remain mute on the subject.
ETA: I'm seeing other posts about how they have to legally allow you to pump, and that's great, but I still wouldn't tell them during the interview. After.