I'm glad this worked out so well for you. Broken fasteners are a real pain. Three comments:
- Very often (almost always?) the application of PB Blaster to a stuck threaded fastener will free it up to the point that it can easily be removed. Sometimes this requires repeated daily applications over a period like a week (especially for fine threads or in cases of significant corrosion). I have a problem like this with a stuck valve stem that I'm just working on slowly over time. I just about always use PB Blaster as a precaution on any old fasteners.
- Remember that the "best practice" in drilling out any broken screw in circumstances like this is to use a left-hand bit in a reversible drill or drill press. This serves two purposes. First, it may end up acting as an "easy out" and actually unscrew the bolt as you're attempting to drill it out. But second (and more important) it will avoid the opposite effect of driving the broken screw stub into the hollow piston -- where it will rattle around for the rest of eternity.
- If you get a fastener partially unscrewed and then it breaks, leaving a bit above the surface, an often effective approach is to use a Vise-Grip of one sort or another to clamp onto that stub and very carefully continue to unscrew it. The Vise-Grip, unlike pliers, won't slip; and you can apply a huge degree of torque to the broken fastener shaft. This is difficult on very small fasteners, but is still worth a try if there's enough of the shaft available.