I hope people aren't sick of hearing about this build! I deeply appreciate everyone's time and wisdom, though; it's amazing how much I'm learning (also shocking how much there is to learn).
I started with an NOS pair of dual pivot caliper brakes, which were a good deal because they're a mismatched pair of Campagnolo Veloce front and Potenza rear.
The brake levers are inverse bar-end Tektros, which I tripled-checked are meant for calipers and not linear/v-brake.
Cables and housing are new of course.
Upon install, the front brake feels solid when you pull the lever. The rear, though, felt "stretchy"/spongy/weak, however you word it; the pads would contact but you could keep pulling the lever until it bottomed out.
I tried the pads closer to the rim, verified that all the housings are seated but same thing.
My first theory was, "could it be the brake caliper arm flexing"? It seemed unlikely, but these are literally the first Campagnolo parts I've ever touched, and the rear brake arm is hollowed in the middle, plus the rear is a different model than the front, so I figured, maybe this is a known issue with this one model of the Potenza only?
So I bought an old Dura Ace rear brake on Ebay just to test, and it felt exactly the same.
My next theory was perhaps the levers, being pretty cheap, have bad tolerances and have play in them, but this theory seems undercut by the fact that the left brake feels solid when I pull it.
It seems to me now like it must be something with the cable housing--maybe it flexes somewhere when I pull it and it loses some tautness. It's a pretty long run, and I have it wrapped under the handlebar tape, which is something I've never done before, plus the frame has internal cable tubing, so I suspect I did something wrong either at the points where the housing terminates in the frame, or in that long run under the bar tape.
I watched the Park Tool videos by Calvin Jones about taping and installing housing, which are excellent and informative, but I still suspect there's another piece of info I'm missing. Pics follow:
