On most CF frames the internal channel for the rear brake cable is designed to guide the cable through and come out at the rear opening without too much fuss.
However, I am working on a frame now where this is not the case. It goes in all the way until it hits something in the rear, but can't seem to find the opening at the rear cable stop despite endless fiddling.
Does anyone know of a trick to solve this problem?

can you hit this obstruction from both sides ?

Yes. I tried from either end, with the same result. It seems the channel does not guide properly.

Remove the seatpost and see if the top tube is open at the seat tube end. You may be able to stick a finger into the top tube and either find the obstruction or guide the cable into the exit ramp. My '92 Trek 1420 had internal rear brake routing and no guide channel. Guiding the cable out the exit hole from inside the top tube was the olny way to do it the time I inadvertently removed the cable and housing together.

You were shipped a 2nd..
Might be s blob of resin in the tube, inside ,
and they should have not shipped out the frame in the 1st place.,
but QC gave it a pass, to meet sales count..

but I'm working blind.. what does the LBS say, when you had them look?

That sounded like a great idea, but unfortunately it didn't pan out - it is not open to the top tube.

I think it may not be a blob, because my LBS originally managed to get a cable through, although they said they had to fiddle a lot with some kind of snare. I just can't think of what to use to snare the cable from the other end.
Haven't taken back the frame to them again. I am kind of stubborn when it comes to doing stuff myself, and thought if they could do it last time I should be able to do it as well. With the help of some experienced advice in here!

Any chance you can try the floss-and-vaccuum trick to bring the cable through?

I'm assuming you're already trying to thread the cable, not the housing.

If that's right, you should use a new cable with its end still welded. They find a path better than a cut cable.

The floss and vacuum is a good idea if the cable won't work.

No, actually never heard of it. I suppose the method is self-explanatory - suck a length of dental floss out the back end while holding on to the front end, then tie the cable to the front and pull through, right?
Sounds like it may give me a chance, if the suction is sufficient to pull the floss and the floss pulls the cable past any resistance.
Thanks, I will try that for sure!

Yep, exactly.

I've found that "Glide" flat floss works best. But just try whatever you have on hand first.

Well, sucking the floss through was not too difficult, but attaching the cable to it to pull it through proves another matter! I tried with all kinds of tape, and now I have super glue drying on it. I can't put too much tape over the wire, to avoid getting it too thick for the exit hole.

Instead of tape or glue, unravel a couple inches of the end of the cable, cut off most of the strands, then try tying the remaining strands into a loop that you can attach to the floss. Alternately, use an awl or nail to spread the strands an inch or so from the end of the cable, and push the floss through the hole and tie it off. The remaining bit of cable after the hole should hold long enough to be able to pull it through.

That sounds like a plan! I was trying to think of a way to tie the floss to the cable, but couldn't figure out how. I also have to be careful not to shorten the cable too much and not reach the brake clamp.

Most new brake cables are significantly longer than needed even to reach a rear brake. I usually wind up cutting off a foot or more of extra wire.

Yes, I know. But this one came still connected to the brifter of a used Rival group I am installing. I may have to replace it for the sake of getting this job done.

most internally routed carbon frames are not lined inside. the top tube is usually pretty open. the port for the cable is usually big enough to stick a magnet in. post some pics of what you are working with

so is the cable tip still welded?. new ones are ..

I try to solder mine, where I cut them, before cutting.

it may be fraying in the housing and jamming..

+1. Difficulties, whether major or minor, in getting the cable(s) through are the norm, not the exception...... unfortuntately.

That is the impression I am getting too. This is one "advancement" we could do without!

It has been cut, so there is no solder left. However, I am always making sure the cable ends are not frayed. I also tried, just to test the channel, to feed a new shifter cable (4mm, nitrite coated!) through it, and could not find the exit hole. Very frustrating technology, seems to be designed to make life miserable for the home mechanic. I am ready to swallow my pride and pay my LBS to do it.