In the late 1980s, I had an '86 Corvette I bought 2nd hand; the brake master was not holding pressure.The problem with remans is almost universal ( and this comes from inspecting a few vendors for approvals on lists as part of reliability programs under ISO- auto parts reman facilities are no different)
These places don't hire mechanics or technicians- they hire workers who can be trained to follow a process and a gauge in an assembly line fashion.
They do these things in bulk and what little testing that's done is often nothing more than an air puff against a spring ( 1 cycle) and if it cycles and doesn't hang- it goes on the belt to packaging.
Most times they don't even evaluate the core, just change parts to a checklist.
I was really surprised at first but some of them are proud of it. I see "quality" signs everywhere.
More of that MBA mindset from the other thread.
There were no rebuilts or new ones available as the brakes were revised in the 1986 cars. Completely different master cylinder.
My older brother rebuilt it; I had my doubts.
He meticulously honed a little; carefully inspecting each iteration.
He told me if he goes much further, it's junk. Finally he declared it good.
The $8 rebuild kit saved the day; braking on that wonderful Vette was outstanding.