try pushing on that piston to see if it moves at all. if it doesnt, then you will have some work ahead of you. soaking it like you are is a good idea, but not sure if brake fluid would be the best to get it loosened. what you could do is used some light compressed air and blow into the inlet port of the master cylinder to force the piston out, just make sure you have a big stack of paper towels or something very soft so when the piston comes flying out, it doesnt fly around. im assuming your master cylinder rebuild came with a new piston like mine did? if so, you will likely have to put the 2 piston seals on and that can be a pain, just make sure you have the lips of the seals facing the right way, if you dont, the master cylinder will not work.
on a side note, i just remembered that you tried bleeding this before and couldnt get it to bleed. what actually happened when you pushed on the brake lever? the spring for the lever would make it return, but if the piston was stuck, it would be stuck up inside the master cylinder which would be why you could not get it to bleed right. its hard to really tell exactly what its doing without actually being there. i just rebuilt mine this past weekend and i used the vacuum bleeding procedure like you had tried, and it worked great for me, only took maybe 5 minutes to get it completely bled. i hope some of this helps, im not good at explaining things. let us know what you come up with.