I'm try to remove the brake-cable port on the top tube of my Fuji Transonic to thread a new rear brake cable through. Unfortunately, the port is held in with an allen bolt and the bolt threads are stripped or the nut on the inside isn't fixed anymore so turning the allen bolt with an allen wrench doesn't bring the bolt out, it just rotates. I've never touched the rear brake cable before so I don't know how this happened. Unfortunately, removing the seatpost doesn't give access to the top tube, it's sealed off. I also can't just thread a new cable through because the port has a very small hole, just barely large enough for the cable to fit through.

Any advice on how to fix this (or just get a new cable through) would be very much appreciated! I'm trying not to take it to the shop because they'd need it for a few days and I'm in the middle of training for a big race.

They make a thin tube you can slip over an extant cable so you can then push the old cable out with the new cable. It's like un-shrunk heat shrink tubing.

Securely attach a strong string to the end of the old cable. Gently pull out the old cable. Detach the string then attach it to the new cable. Use the string to pull the new cable into position. Simple, yes?

Sure... If you can tie a small enough knot to fit through the housing stops.

Problem is, the old cable is out. I didn't realize the problem with the bolt until I'd removed the old cable. And how would you securely attach a string to the cable? I've never understood how to do something like that.

Any number of ways to attach a string. If the cable end is soldered, just pass the threads between the cable strands and double back the string. Or, cut back half the cable strands, then bend the others back to form a long hook. Loop the string through and flatten the hook. Maybe glue. Maybe learn how an electrician uses a fish tape.

At the end of the day it only matters that you get it done. The cloud can't do it for you.

Ok, just use a full-length brake cable and housing zip tied to the outside of your frame.
Forget about threading the internal routing for now.
That'll get the bike rideable for your training/race/event

I freaking hate internal cable routing. My Cervelo has a FD hole so small that even Park Tool's 'internal cable routing kit' doesn't even fit through it.

Suggestions above for attaching a string to a cable. A vacuum cleaner will draw the string through the exit hole.

Aha! Brilliant. I like that idea a lot. You could use monofilament.

Thanks everyone. I'm going to try the vacuum cleaner idea, just haven't gotten access to one yet. Good suggestions on thread attachment, I hadn't thought to interleave with the cable strands. And yes, the backup plan is an external cable. I guess there's no magic trick to this I hadn't seen before.

I like the fishing line idea, thanks!