Should be explained by fluid being pushed back in to the reservoir. As it does so the diaphragm bulges out and pushes air under the cap out through a vent.
Sounds right Howard, thanks a lot. I arrived at the same conclusion late last night, the noise suddenly became audible because of the excess brake fluid in the vent. I expect it'll go away eventually.
Should be explained by fluid being pushed back in to the reservoir. As it does so the diaphragm bulges out and pushes air under the cap out through a vent.