Penetrating oil, heat, and vibration...that's about the only things we have besides brute force. Soak it...vibrations...heat...soak...etc. Wackling on it with a brass, wood, or plastic mallet is good for vibrations...not a mateer of trying to beat it to death, just good solid wacks to vibrate it a bit and perhaps get the oil where it needs to be.
IF that fails, then BOIL the cylinder...that's right, plain old boiling water. Will get the temp up to a safe 212degrees and boiling will loosen the rust, congealed grease, and fouling holding the nnipples in...then dump in the penetrating oil into each cylinder and let it sit.
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While you are waiting arround, may as well go for a full dissasembly...chances re, if he let the nipples rust in place, the lock work hasn't had much attention over the years.