A brake cable tunnel through the top tube was an optional extra that I offered initially on my custom frames, it included a shield shaped re-enforcing plate where the slot was cut in the tube.
This thin steel plate brazed in place was mainly for decoration, and was often chromed as shown here. Structurally it was unnecessary and later when I offered the tunnel as an extra on the Fuso I dispensed with the little plate.
Had I simply cut slots in the top tube to thread the cable outer housing through, then some form of re-enforcement would have been necessary, as the top tube would have been considerably weakened by cutting slots in it.
I did not like this approach as moisture can enter through the slots causing corrosion inside the top tube. I made my tunnels a self contained and sealed unit.
I did this by first cutting the slots. Then I took a thin steel tube with a hole just big enough for the brake cable inner wire to slide through, bent it to a curve, then after measuring the length I brazed a short piece of a larger diameter tube on each end of the first tube.
This assembly was then threaded through the slots and was brazed into place; in the case of a custom frame the shield-shaped decorations were added at this time.
Then with a hand held drill and a drill bit just slightly larger than the outside diameter of standard brake cable housing, the tunnel entrance was drilled out from either end. See drawing below.
Finally, the surplus tube was cut off and filed flush with the outside of the top tube. Or in the case of a custom frame, flush with the re-enforcing plate. See the following illustration.
With this system the cable housing went from the brake lever to just inside the front end of the tunnel; the bare wire went through the thin steel tube, and another short piece of cable housing went from the rear exit point to the rear brake.
It did however call for some maintenance. Although no moisture could enter the top tube, the cable entry point could collect water as it drained down the outer cable.
Customers were advised to keep both the inner wire well greased to keep friction down, but also to grease the cable housing where it entered the top tube, to keep moisture out.
Even on the custom frames with chrome as shown here I would still advise plenty of grease at the entry points.
Unfortunately I have noticed some frames with tunnels that have a creeping rust problem from inside the hole to the outside surface of the top tube.
This would have to be pretty bad to become a structural problem, because of the way the tunnel tube was assembled as described here.
If you have this problem, unless you are planning on a complete repaint; I would advise to clean the surface rust down to the bare metal, treat with Navel Jelly, and touch up paint.
Let the paint dry thoroughly, and then apply plenty of grease as previously described. You might also consider using some clear silicon sealer at the entrance point. (The type of sealer used in kitchens and bathrooms.)
Fill the entrance hole at both ends with silicon; assemble the whole thing complete with inner wire. When the silicon is dry, remove the inner wire, grease thoroughly and replace.
But remember, paint and silicon don’t mix, and neither will silicon and grease, so take your time and do one step at a time. Allow paint to dry, then allow silicon to dry and then ad grease to the inner wire.