For 1/25 i've used the inner wires from old USB cables, they're thin enough for HT leads etc.If you have an old Playstation controller then the wires from this seem to do ok for the battery (there's normally a black and red coloured wire)
For any brake pipes etc i've used fuse wire as it comes normally in three ampere rating thicknesses on a little card.
I've only made a couple of cars but this is what i found to be adequate for the job.
I've been using the wire from old telephone wire, which can look a little thick, I never thought about using USB cable, many thanks for the tip Sir, will give it a try next time, as for brake lines all I use is the inner copper wire from normal wire...