What was described with the master lock is likely either rust or some foreign object obstructing the locking dogs, not the cylinder. this is either some thing that got in the shackle hole when the lock was open and laying in the dirt or someone may have put a toothpick or other deliberate obstruction in it. The evilest plug turning obstruction that you will ever come accross is a broken spring that has worked its way past the top pin and fed its end into the gap between the plug and cylinder this only gets tighter as you force turn the plug.
Maybe we should have a thread for malfunctions that the locksmiths would like to read, like when the dead bolt is closed on a short tailpiece/bad installation/wornout lock and the tailpiece no longer communicates with the bolt. its on the third floor apartment, so no real possiblity of going in the window, its either the sawzall to the door or an attempt to operate the thumbturn on the inside with an under the door lasso tool if the thumb turn still is connected to the bolt, A problem like this came up for me several years ago, and it turned out that the boltwork itself was the problem, the pawl that falls behind the bolt was mounted in some zinc casting that had broken. the pawl that drops behind the bolt to "dead' it and the pieces of the casting had fallen inside the bolts chassis.
Should we start a thread on really difficult malfunctions?
call me a butcher i know thank god it was only 99 cents and i picked up 5 