That's all well and good if the OP knows exactly how a brake master cylinder works and more to the point, what to look for when he's got it in bits in front of him and how to go about fixing or condemning it !
The fiddling about, as you call it would take at most 30 mins, a miniscule amount of time compared with the hours that have been thrown at it to date.
Making the plugs, there are other ways to plug them a bolt and PTFE tape,
would do and allow, as well as testing his original cyl, allow him to prove say a second hand master cylinder, or a rebuilt one is up to it's job before refitting and finding, that without testing it, the problem still remains.
Yes many go down the ' easy ' path, but as often seen on here and elsewhere, the ' easy ' way, in many instances doesn't result in a fix first time, resulting in more hours wasted.