rear master cylinder reservoir removal.

I have seen a lot of race bikes with it removed and a piece of light green/blueish but see through pipe with a bung in. Is this a special item or 'just' a clear pipe with a bung in it?

That's HRC pipe. Brake fluid proof and more expensive than Unobtanium!

I used straight forward braided clear stuff. Make sure the bung allows the tube to breathe to prevent a vacuum when using the brake. Or just leave the std one on which adds 0.0005 grams to the bike.

Does it have to be clear so scrutineering can see fluid in it or can you just hack it off and stick a bung in? I know you wouldn't see level but you would see a leak early with all wheel swopping on a race bike plus I don't use mine.

Road Race Developments do them if you want part with 10 quid
I just shortened the original pipe used the same reservoir and mounted it under the set on the subframe mount

Be careful if you replace the pipe with a different tube as brake fluid is nasty stuff and can corrode some plastic pipes. The 'HRC' style tube is safe to use but does sweat a bit and it looks like it's leaking but its not.

I've found that fish tank pipe works wonders, and if your really nice you could ask for an off cut and it will be free. Garden centres like world of water do it, or maybe a pet store.

How on earth did you figure that out? Lol. Did you accidentally poison your fish?

My other half's father works at a world of water, and he over heard me saying I need pipe and he thought of it, brought some back and two trackdays, One race later it's still there.