I have an no-name time trial bike from the 90s (track ends, rd hanger, someone on C&V suggested it might be an older time trial bike) that has internal routing for brakes, gears, etc.

The problem I'm having is that the guides in the top tube for the rear brake are too tight for 5 mm housing. I can push the housing in about an inch, but applying the brake causes the housing to start to shred against the edges of the "port" so there doesn't seem to be a "stop" inside the top tube.

Using any sort of step down ferrule also seems out of the question because the ferrule itself would "float" along the edge of the "port" <-- does this make sense?

It seems like the internal routing for this frame requires cable housing < 5 mm... of which I've only been able to find the compression-less variety.

I've currently "solved" this by stripping the plastic cover from a length of housing which allows it to fit through the internal guide and then using double ended ferrules to complete the housing runs to the lever and brake. It works, but appearance-wise, it's not idea.

Is there brake cable (or non-compressionless) housing that is less than 5 mm in diameter? Has anyone else come across a frame with internal routing like this (e.g. odd diameter guides)?

Thanks for any help/suggestions,
James