Standard 'bundy' brake piping - but use KUNIFER pipe, it doesn't rot and certainly NOT Copper! Front end and master cylinder are 3/8" UNF thread - rear axle (for some mad reason) is 3/8" BSF thread.
Copper work hardens, and can therefore fail, but I've yet to have a problem with the copper alloy type. I'd use Kunifer, but I find it very hard to make the pipes up as it's so hard, Must sus it out....
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Copper is perfectly okay to use. However there is copper and 'copper', ie pure or poor. Go for British made pipe from the likes of Automec, very good quality and reasonable price.
beware pure copper -it work hardens !! Copper 'alloy' may well be kunifer, cos that's what it is (CuNIFe) - which is the recognised best standard for long lasting brake pipes.
And yes - the flexi to the rear axle is a strange beast with 3/8unf at the pipe end - and 3/8" bsf (which is coarser than unf) at the T end on the axle!