Hi

Interesting problem !

years ago the master cylinder would incorporate a valve to maintain a residual (positive) pressure in the system to prevent air being drawn in. This problem was associated with old style CUP seals.

As far as I am aware current system with spigot mounted seals don't use this.

I have experienced similar fault with worn seals on cylinders, but this should not apply to your case.

It would be worth checking the vent on the master cylinder in case a slight vacuum is formed, however I find it difficult to believe it would have any effect on main caliper type seal. (h/b pivot seals are less robust though )

Cheers Aidan