I'm overhauling a vintage Campagnolo caliper brake and the spring is extremely stiff. Unfortunatley I can't find new springs that fit (some almost fit, but are too short). Thoughts?
I redid a shimano caliper brake recently, and on reassembly the spring was massively too strong.
Turns out I'd overtightened the pivot points and they were resisting in both directions. Check that the main bolt is not too tight.
Also look from the side and make sure the spring is not flipped and pressing on the metal adding friction.
You should be able to squeeze the brake closed with one hand using light-medium pressure between one finger and thumb.
I see three possible solutions
find replacement sprints that have the same wire diameter (so they mount in the holder/slot) and the same general shape but are softer. You might be able to salvage some from any other caliper brakes, but the leg lengths may be hard to find.
Squash your current spring a little using brute force, and perhaps some heat to anneal the metal. There's a big risk of overshooting the mark and ending up with insufficient tension, and having to go the other way. Risk is high.
Make some new springs using the current ones as a pattern. Again this is going to be a lot of work, heat, bending, tempering and annealing. Risk is even higher than bending the current spring without snapping it.
You might consider making some kind of "standoff" that lets the brake spring sit under less tension, but that's a horrid idea too. Brakes just have to work.
Personally I'd rather have reliable working crappy brakes than something cobbled together.