Well, I never did figure out why the cables didn't fit. I rebuilt them instead.
I have access to 4mm SS cable and hardware normally used for commercial grade railings. It's thicker than the brake cables, but they still fit through the sheath nicely. I did have to drill the end ferrule bushings of the casing but that was about it.
Then I had to hunt around for a return spring for the drum end since the one supplied was too short and too stiff. Some old spring-loaded desk lamps fell victim to my cannibalizing (pack rat FTW!). Once I rebuilt the passenger side, I decided to rebuild the driver's side too. One side got the "updated" spring while I tried to use the supplied spring on the other.
Everything works nice and smooth now but the passenger side ('lamp spring') locks up before the driver's side. Turns out desk lamps used good quality springs...who knew?
I'm a little disappointed the brake cables didn't work properly right out of the box, but I ended up with a better result anyway.
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