Mercedes W123 200 – Brake pedal hard

Hello dear car friends , I hope with my request here right I bought me a 123 half a year ago, got on the first day immediately TÜV and so far I drove only 50km. PROBLEM: I noticed now several times that the brake pedal is occasionally (until often) very hard, can hardly be kicked and the car then has hardly any brake force. The pedal can be moved zero, I thought maybe also the discs “glazed”, because it feels like it would disappear after a while. I changed engine oil and brake fluid last month and had hoped that the brake fluid change is away from the problem. (air in the line o.ä.) Was now in 3 workshops, which were rather asked or had no desire for a vintage car Can someone give me advice (clear is difficult over ́s Internet) where I can start or give an approach to what this can be? I thought about the brake power amplifier, could it possibly also be sold out or had no desire to do what disks with hard pedals now – but I am also not an expert and I am happy about it ^