Possibility 2, during the "free" bit at the start of a Shimano lever stroke, the valve between the master cylinder and reservoir is open. If your reservoir level was quite low and the seal at the reservoir port is better than the caliper seal, the pressure difference between ambient (inside the reservoir) and the lower pressure of the hold (higher than atmospheric at altitude, but lower than sea level) could force a little fluid past the caliper seal during a flight with up to 2 hrs cruising.
Don't know enough physics/engineering to calculate the likely seal leakage for a given pressure difference though.
Howard
coventry_eagle
Possibility 2, during the "free" bit at the start of a Shimano lever stroke, the valve between the master cylinder and reservoir is open. If your reservoir level was quite low and the seal at the reservoir port is better than the caliper seal, the pressure difference between ambient (inside the reservoir) and the lower pressure of the hold (higher than atmospheric at altitude, but lower than sea level) could force a little fluid past the caliper seal during a flight with up to 2 hrs cruising.
Don't know enough physics/engineering to calculate the likely seal leakage for a given pressure difference though.