breaking fluid contamination problem

Brake fluid that's many years old will be brown so don't let the color make you change all the brake seals etc. The old brake fluid can easily be flushed out and new fluid installed for moderate cost.

Get another opinion.

I think you need to find another mechanic... all you need to do is flush the brake fluid... very common practice and should be done every two years. If he is trying to trick you into thousands of dollars of repairs... he is a crook.

I guess the question is whether it really is contaminated with anything to begin with. Do you have any reason to believe anything other than brake fluid was put into it? As others have pointed out, if it's just very old brake fluid, it will turn darker over time. When the guy said the whole braking system needed to be changed, did you ask him exactly what he meant by that? In defense of the mechanic, if he good reason to believe it was contaminated, the way liability works today, I could see where he might want to be overly cautious regarding recommendations. If someone came in with a system full of water and God knows what else, and all he said was change the fluid, I could see someone later having a brake lock up, accident, sue the guy, etc.

If the brake system is otherwise functioning, OK, I'd just flush it. How well you need to flush it would depend on whether it was contaminated with some other material. If it was, then I'd probably pull the calipers, force the pistons all the way in, to get all the old crap out. Just trying to use the bleed screw, IMO, is fine for replacing fluid, but if it was contaminated for sure, then I'd want to get it all out.