Reply to "Brake master cylinder"

Steve is correct- in the old days, we routinely tightened a sheet-metal screw into the brass press-fit flair-seats of such master cylinders to easily pull them out with pliers. Behind it was what we called a 'duckbill'-valve which could then be pulled out with needle-nose pliers. They were often found damaged so they didn't work anyway. IMHO this valve was put in drum-brake circuits of both drum-only and disc-drum combo cylinders. The keep-alive pressure in drum brakes is about 12 psi while disc brakes use a 2 psi valve, so you certainly don't want to use the wrong one! On the GM master cylinders used in Pantera conversions, the 2-lb keep-alive valve is a thick nickel-sized rubber thingy with a tiny coil spring & a ball bearing for the check-valve, installed under one of the the plastic fluid reservoir spouts.
I personally don't use such valves at all, since they cause a small amount of pad-drag as the price one pays for getting that theoretical 2-millisecond-decrease in brake response time. Our modified brakes can lock wide tires very quickly when panic-stopping, so it's not needed, and my pads & rotors live longer without it. But then, the only stock parts of our brakes left are some of the hard lines.... so YMMV.