Mechanical disk seizure
Six years ago, I invented a thing called Dryline. My back brake, a mechanical disk, had shat itself. It wouldn't operate from handlebar, but could be operated at the brake caliper, itself. Had to be the line. It was, after a few hours of googling, I discovered this is a problem with mechanical disk brakes mounted inside the rear triangle. The cable fills up with water and road grime in winter and, unlike the old days, when bikes were designed with sheath sockets and open cabling, but disk brake frames have to make way for hydraulic lines, so by extension, mechanical cables have to be sheathed all the way, too.
The problem is solved by installing a "cable vent" below the caliper, but above the bottom bracket. I couldn't find such a thing, so I invented one. I designed it, boaught a 3D printer, and made several, before settling on this design. (I'd forgotton the old cable runners that used to be under bottom brackets, BTW, that's another possible solution, if you can fix a cable socket at each end of the chosen line run.)
I tried hawking a few around bike shops, but most bike shops (a mate's excluded, ta Matt, appreciate it) would rather sell you a couple of hundred dollars worth of hydro brakes than a $15 plastic part and I like my master cylinder, hybrid mechanicals, so I print them for myself and friends. Today it hit me, all I needed was some M6 internally threaded hex boss, a pair of socketed hollow M6 screws and a pair of zip ties.
I out mulleted myself. Sometimes simpler is better. Do your research (and jeez I hate how that has become a "cooker" term!), there are probably simpler solutions than a 3D printer and a room full of plastic. However, if you need one and don't want to file drain holes in some hex boss, download the STL file here and get it printed somewhere, like at a mate's place, not mine, I'm taking a break from printing for a few months.