Is it true that you shouldn’t turn a bike with disk breaks upside down, and if so why?

Neil

No it isn’t.
Long-term storage can result in needing a re-bleed, but the same goes for being the right way up.

If you happen to have air sitting in the lever’s reservoir (usually due to poor bleeding), it’ll be quite harmless until you leave the bike upside down, when it’s possible that the air could find it’s way into the pressurised side of the system and cause a spongey/long travel lever.Pulling the lever will increase the chance of this happening.

Cheers Guys

Guess I can still turn the bike upside down to change a wheel without worrying.

Neil

occasionally your brakes can feel dead after transporting in a car (rather than just popping upside down), if it does happen: just grab the lever and hold it engaged for a couple of minutes, release and repeat until the feeling comes back.

all my bikes are hung by their front/back wheels when not in use and I’ve never had this problem.

I have read that Avid’s are fairly prone to getting air in the system if not stored ‘right side up’ though.