Well, usually one just disassembles the brake components and cleans them out (the master cylinder and the caliper). But like I said, I don't think you're equipped to do that. The master cylinder requires some long reach snap ring pliers to get it apart and the caliper usually requires compressed air to blow the piston out. I've renovated quite a few of these and usually just a disassembly and thorough cleaning is all that was needed, no new replacement or rebuild parts.
Like I said, yours looks pretty bad. There's a good chance the piston area below the reservoir you've looked into is all packed with gunk. It probably isn't pumping fluid like it should and that's why the brake is so weak. It's barely being applied. As also mentioned, a basic fluid flush and change usually won't clean all the crap out. So, you have a decision to make - attempt to renovate what you have or buy a new MC. But that won't address the possible caliper issues. Eventually you're going to have to take that apart and clean it out. Eventually it will start making the brake pads stick and drag. Don't be like a local guy I know. He let his get so bad you could barely push the bike around, lol. A complete disassembly and cleaning of both his MC and caliper had the brake working like new again (no new parts needed).
One more thing I should mention. You'll want to replace your brake line with a stainless one. They're dirt cheap on eBay now, less than $10. A 90cm long line is perfect for lower bars.
Like I said, yours looks pretty bad. There's a good chance the piston area below the reservoir you've looked into is all packed with gunk. It probably isn't pumping fluid like it should and that's why the brake is so weak. It's barely being applied. As also mentioned, a basic fluid flush and change usually won't clean all the crap out. So, you have a decision to make - attempt to renovate what you have or buy a new MC. But that won't address the possible caliper issues. Eventually you're going to have to take that apart and clean it out. Eventually it will start making the brake pads stick and drag. Don't be like a local guy I know. He let his get so bad you could barely push the bike around, lol. A complete disassembly and cleaning of both his MC and caliper had the brake working like new again (no new parts needed).
One more thing I should mention. You'll want to replace your brake line with a stainless one. They're dirt cheap on eBay now, less than $10. A 90cm long line is perfect for lower bars.