When I first get going in my car the break pedal jolts/clunks. Only once p/drive

What is your question?

I bought a used 2005 bmw 325i and I have started doing to notice recently that every time that I start up the car and drive about 50-100 ft the break pedal reacts as if I ran over a pothole or something similar, it's only when going downhill too. Any info on problem/fix/repair cost, would be greatly appreciated.