Mercedes W123 200 – Brake pedal hard

Hello dear car friends , I hope I am right here with my concern I bought me a 123 half a year ago, got on the first day immediately TÜV and so far only 50km ridden. PROBLEM: I have 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 braking power. The pedal can move zero, I thought maybe also the discs “glazed”, because it feels like this and would na I changed engine oil and brake fluid last month and I had hoped that the problem was away from the brake fluid change. (air in the line or similar) Was now in 3 workshops, which were rather over-examined or had no desire for a vintage car Can someone give me advice (clear is over’s Internet difficult) 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 a block e/Displaying? Didn’t know what to do with hard pedal discs now – but I’m not an expert either and we also notice you’re inquisitive / desperate ^^ Thank you and I’m looking forward to your approaches