Bubble??

[Fred1956]

Might be a silly question but I'm trying to wrap my head around it before I get to work on it....2013 EC300 with spongy front brake lever & poor braking. Short of time on Sat but I back bled it from caliper up and noticed what looked like a ball of water in the bottom of the master cylinder, turns out there is a clear 'dome' over the feed holes for the plunger & it was holding a big air bubble over the feed hole, but I couldn't figure out a way to get the bubble to float out... Any ideas? Anyone see this before? I tried to gently pry the 'dome' out but didn't want to be to rough as he had a race on Sunday. (Won the Pro class anyway) Advice or thoughts are appreciated. Thanks