I would like to transport my MTB on an airliner's cargo hold. What are the requirements concerning hydraulic brakes, do they need to be flushed?
Tires on planes are a problem because they are pressurized above sea level atmospheric pressure. The pressure on a commercial airliner drops below that at sea level, so the pressure difference increases which can exceed the pressure the tire can handle.
Hydraulic brake fluid is not pressurized above sea level atmospheric pressure and the brake system is designed to withstand high internal pressure when the levers are squeezed anyway.
I've heard of airlines requesting that you deflate your tires, but I've never heard of people draining the hydraulic fluid from their brakes.
Two friends of mine recently flew from the US Pacific Northwest to US Southwest for a week-long mountain bike trip. They flew with their bikes as checked luggage. By their accounts, they did not need to deflate their tubeless tires (thankfully), their fork or shock, and they did not have to drain the fluid from their hydraulic brakes.