Brake fluid loss

I have a 99 dodge ram 150 4x4 with 110k miles. At about 80k I had the brakes replaced. Rotors/Drums included. Problem is now I am having to top off my brake fluid about once a month. Can't find where the leak is. No sign of leakage around calipers or on brake lines or reservoir. Any ideas???????????

Do you have rear drum brakes? If so, have you taken off the drum and inspected in there for fluid?

Barring that, put some clean cardboard under each wheel and let the vehicle sit overnight, look for leaks in the morning. Also run it for awhile in place, and pump the brakes several times.