it is a easy swap and yes it could very well be a POS rebuilt master cylinder.
Here's the thing... A master cylinders piston rarely if even travels the length of the bore. The short stoke of the piston wears the bore in only that area.
Reman'd could test good before leaving Coolios hands and fail again during bench bleeding.
I honestly dont understand why people will buy a rebuilt master cylinder, especially one that might be 40 years old and on its 3rd or 4th recycle.
But anyway... good luck with it.