I have done this procedure several times on completed frames, but it is sensible to do it before the tube is brazed. Partly because it reduces the chances of frame distortion when you are brazing into tubes after assembly and partly because if it goes wrong it will be easy to re-do! I have only used brass tubes for this procedure, though it would be possible to use stainless tubes if you can obtain malleable enough ones of the correct diameter. Ceeway and various model making suppliers supply 7mm diameter brass tube with approximately 0.4mm walls. You need 500mm lengths to do the job. A 7mm tube will take a complete brake cable, holding it tightly. It would be possible to forgo the internal tube and simply run the cable in and out of the top tube but I suspect this may cause unacceptable noise from the cable being free inside the top tube, and allow water ingress.
It is not, of course, possible to brass braze in brass tubes as they will melt at brazing temperature so this procedure requires silver soldering and care not to overheat and melt the brass tube.
Running brake cable tube into the top tube is not difficult if the in and out ports are on the same side, conventionally on the left hand side. The technique is well described in the Paterek Manual and is as follows;
Select the positions in the tube for entry and exit, usually about 8-10cm from the ends. Drill a pilot hole vertically followed by a hole the size of the guide tube, i.e. in this case 7mm. Then using a long shank drill insert and angle the hole in the direction of the guide tube i.e. inwards into the top tube. Using a standard length drill means you would be unable to angle far enough without the chuck of the drill striking the tube.
This leaves you with a hole like this. In fact I was unable to find long shank 7mm drills, only 6mm from my local Screwfix, though they are now stocking them. I have managed thus far using a 6mm drill and filing the hole to fit with a half round or oval miniature file. Some filing may be necessary anyway to make sure the brass tube is a tight fit but ideally not so tight that you cannot draw it out if things are not going smoothly. Drill a similar hole at the opposite end of the top tube on the same side, facing the opposite way.
Now bend the end of the tube slightly towards the insertion and exit sides. The tubes buckle easily if bent sharply or too far. A brake cable sheath inserted will act as a protection against this.
Now gently but firmly push the brass tube into either hole. Then consider pulling it out again to check if you can! otherwise keep pushing. The tube will hit the opposite wall of the tube then curve back towards the second hole as you keep pushing. Light tapping with a soft blow hammer will aid the process. Keep an eye on the second hole for the end of the tube to appear, easy if you have not yet assembled the bike. Poke a thin screwdriver into the second hole to catch the end of the brass tube. A No 2 Phillips type screwdriver is just the right size and will act as a guide to direct the tube out of the second hole with some encouragement from the hammer at the other end.
The tube ends can now be silver soldered around the bases then cut off and filed down flush with the tube.
I prefer to silver solder a covering cap over the hole though some framebuilders leave the hole visible. Remember to insert a cable into the hole before soldering to ensure the cable cap is in the correct position.
FITTING TOP TUBE GUIDE ACCROSS OPPOSITE SIDES OF THE TUBE
There is a current vogue for routing rear brake cables from the front right side to the rear left side of the top tube. This is much more difficult and the method I describe is how I have tackled the problem. Improving on the only partially successful method with the bike assembled this method depends on the procedure being carried out on an unfitted top tube.
Firstly drill and angle the holes where desired as described above on opposite side of the top tube. This is obviously best done after pre-assembling the frame to ensure the correct positions. Place a slight curve in the guide tube as before but angle to the opposite side and begin insertion.
As the tube hits the opposite tube wall it again curves back on itself but remains mainly along the opposite wall and it is extremely difficult to catch and bend the tube through the other hole. The tube ideally needs to be feeding through the centre of the top tube. I got around this by inserting a 1metre length of angle iron (available from your local B&Q) as shown along the opposite wall between the wall and the brass tube levering it (upwards in the picture above) by lifting it from both ends of the top tube. Do this in stages as you tap the tube through by inserting the angle iron, lifting it, removing it, tapping it through a bit more then re inserting the angle iron and repeating a few times. Leaving the angle iron in place and trying to push the brass tube all the way through sounds like a good idea but I found it too tight an angle to work. This procedure has the effect of straightening the tube as it enters the insertion hole and give it a more central position. Care still needs to be taken to avoid kinking it.
You can also use the tip of the angle iron to act as a pivot to curve the end of the brass tube further through the hole. You will likely need some help from a hammer at the opposite end.
Check a cable will run through OK before soldering it up and putting it aside for later
I would like to hear from anyone who has any alternative, hopefully easier methods of internal cable routing