Come on, there has got to be a reason somebody knows? 
I think it is a vent so the badder can expand and contract correctly. Some times brake fluid gets by the bladder if it is not sealed correctly or their is a tear in the bladder. The bladder should be sealed with Parker O-ring lube between the contact surfaces of the magnesium housing and the bladder.KCC said:Come on, there has got to be a reason somebody knows?![]()