There needs to be a little air above the fluid, like the free space in a zero pressure radiator tank. The first thing that happens as you push the pedal will be a little bit of a fluid surge back into the reservoir before the piston starts to pressurize the fluid in the lines. Overfill the reservoir, and the surge will push some fluid out of the vent. Try using a turkey baster or something similar to remove an ounce or two of fluid- - - -I'll bet the problem goes away.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!