Master cylinder malfunction

[1976RD400C]

So, I put these rear sets on, that have a master cylinder mounted right on them, a couple years ago. Everything was going ok until a couple days ago and 3 miles from home the rear brake starts dragging. I have noticed the rear petal was always pretty firm. I kept going and it keep getting worse. Got home, just kept giving it more throttle. The rear pads were smoking. What the?? Checked if the push rod had free play, yes, but the master was bound up tight. Pulled everything apart and rebuilt the caliper. Torn down the master and it looked clean and ok. Checked if the "little hole" was clear. What the?? Let me get a good look at this. There was no little hole. I see the set screws and plugs outside the bore that are there when they make the thing to drill the 2 holes into the bore, but the little one was not finished with the small hole to the bore just ahead of the piston cup. Used a .75mm drill and Dremel. All good now with normal petal travel and release. How it didn't cause trouble earlier is beyond me. Major quality control problem. China, China, China.   

[m in sc]

oof!