Copper brake pipes contain nickle to prevent issues with vibration, the stuff is designed for the job. If it was pure copper it would crack.

Metric and imperial pipes may be close but for a safety item like a brake pipe close is simply not good enough. You need to get the correct size and fit it with the correct unions.

But there may be manufacturers who do pipe/union combinations that are designed to work perfectly but I have always bought the correct size.

Brake failure is not an option.

Silicon brake fluid is not a solution in any car.

Is there anything like an MOT where you live?

Sort of, but functional test only. No structural inspection, so much more limited than an MOT.

The test fee is less than a British MOT (about 15 quid IIRC) but the inspection probably doesn't cost much to do (emissions, brakes and lights. No bodywork or suspension checks AFAIK) so its probably quite profitable.

For the routine 6-monthly inspection, you have the option of a licensed, privately run test centre. These are quite a lot more...er...relaxed than the govt. centres, probably because they want the repeat business.

For the last-but-one inspection I was having trouble with the carb. The idle was far too high, and I don't think they even pretended to check emissions, but it still passed.

If you need to change the registration, for example, on sale or purchase, you need a govt. inspection. I had to go through that a couple of years ago because I lost the reg. documents, and it was tricky to get an emissions pass. I suspect it wouldn't get one now.