Brake pedal is hard to give in. audi 100 type 44

Hello friends, I have a little problem with my Avanti.. From time to time it is so that when the indicator lamp lights up in the middle and I step on the brake then the pedal does not give in. I say I can climb on it with both feet and it happens quite little. when I stand you notice how the pedal slowly gives in trembling. Stand today on the mountain and wanted to look at how the Audi is pushing for snow, then the flashing started. Stop briefly, but now it comes all the time…. D Every time I get in there is something new. First brake in the rear, then servo pump, transverse handlebars, V-belts……. soon the car is completely renovated. Oh yes, it’s an Audi 100 Avant type 44 Quattro 2,3E. Built 87 I might have thought of the brake power amplifier, the car was probably 6 years in the barn. Maybe there was water formed there so that the thing now has one away. Is there something with repair, or has to install a new part n ?? If yes, what is the fun and you get the still ???? many thanks for your answers already in advance…. mfg tobias