Build a hypothetical post-apocalypse bicycle

Why put brakes on there? And why disc?

If it's fixed you could just go brakeless. It is post-apocalyptic, roads will be largely empty or there would be debris and you're going slow anyway.

Disk brakes require too much precision, and you really wouldn't want hydraulic as we've already disposed of the need for a pump.

Toolless would be ideal, but the fewest tools that you could reasonably find in any abandoned house should be the next goal. It shouldn't be assumed that you would have access to your multi-tool.