Brake problem

Actually you CAN get air into the system without losing fluid. Air can be sucked into the lines before there's a big enough gap for fluid to get out. It also depends a bit on where the leak is. It's not common because when the system is under pressure you can usually force brake fluid out, but with a small enough leak it's possible. And if the brakes last for a couple weeks before enough air gets pulled in to notice you're looking for a pretty small leak.

It could also be a problem with master cylinder seal, in which case the whole issue could be internal to the master cylinder, and you wouldn't find a leak outside in the system. New parts don't always mean good parts, there's always a percentage of new stuff that has issues.