As someone who has never had a bike with internal brake cable routing (in this case through the top tube), I bought an '87 Panasonic DX 3500 bike for restoration.
The frame as well as many of the components are made by Tange, a Japanese manufacturer.
Before removing rust and touching-up the paint, I am going to strip the frame entirely and that would include the black plastic guides the brake cable enters and exits the top tube through. (BB and headset need replacement, so I would have stripped the frame nonetheless.)
The outer cable is routed through said guides; pulling it on each side makes it move on the other side as well, which means and the guides are attached on the top tube like shown on the photo below:
Having broken hard-to-replace plastic components before, I really don't want to use any force before being sure that's the only way to do this.
I would expect there to be a release pin on the cable guides, but I haven't found any.
Does anyone have an idea about how to remove the cable guides — and in case of emergency, knows how to find a new replacement or if they are non-removable, even for maintenance?