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Greg,
There are two holes between the bore and the reservoir. The front hole is the compensation port. The purpose of the compensation port is to "compensate" for expansion of the brake fluid as it gets hot. The compensation port is covered by the piston cup within the first 1/16" of movement forward. That seals the pressure side of the piston and bore from the reservoir. During proper operation the compensation port "sees" only atmospheric pressure and fluid expansion. In order to pressurize the reservoir, either the cup on the front of the piston is damaged or the bore is damaged.
Disassemble the master cylinder and closely inspect the cups and the bore. Do you see any damage to the front cup? If so, there will be a corresponding sharp "something" in the bore. It might be a piece of grit left from honing. If you think you need to hone the bore again, don't. Instead, judiciously apply an emory cloth on your finger. Remove only as much material as necessary to smooth the bore. If you hone the bore again you run the very real possibility of oversizing the bore so that your cups won't seal. That would require a rebore and a larger piston and cups to fix. A larger bore would mean more pedal effort to achieve the same level of braking. Be sure to clean everything thoroughly before reassembling with new cups. If the cups are OK then...
A third possibility is that the master cylinder sat for a very long time unused and is so badly corroded there is a path for the fluid etched into the bore. If that is the case, the master cylinder cannot be refurbished and must be rebuilt with a sleeve.
The other hole between the bore and the reservoir is the transfer port. It's job is to "transfer" fluid to the back side of the piston as the piston moves forward in the bore to prevent suction behind the piston. The tiny geyser that erupts in the reservoir as the pedal is released is the transfer port "transferring" fluid back into the reservoir. This port "sees" only low pressure and will not cause fluid to push past the vent in the cap.
I hope that helps some. Do let us know what "the" fix turns out to be.
--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL
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