Turning a bicycle with disc brakes upside down? Yes or No?

It is often said that you should not turn a mountain bike around. What is the truth of this?
When you turn the bike upside down, air may rise from the brake lever into the line, because the disc brake system is an open system. If you turn the bike upside down again, the air remains in the system and restricts the braking effect.

The brake must be bled.

Never pull the brake lever when the bike is turned upside down. A rubber band around the lever and handle can be used to seal the hydraulic system before turning it over, thus preventing air from entering.

Air bubbles can accumulate in the reservoir of the brake system when the reservoir cap is fitted or through prolonged use in various parts of the system. The M755 disc brake must not be reversed, otherwise the air bubbles from the reservoir may enter the caliper. If the bicycle is used in this condition, there is a risk of brake failure.

After the bicycle has been reversed, the brake lever must be operated a few times to check that the brakes respond normally.

Excerpt from the Shimano brake system installation and maintenance instructions

Nevertheless:
A perfectly tuned and bled brake system is 100% free of air pockets. Thus, “upside down” is normally not a problem

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