Take a look at the calipers as the brake pads engage the rim. Some actually flex as you squeeze the lever tighter. Obviously, if the caliper arms flex, and lack rigidity, they can't squeeze the rim as tight, thus they can't brake as well. This flex is especially noticable on my '62 vintage Weinmann 730s on my Continental and the 800s on my '66 Collegiate.

I added the Tekro areo levers to my Traveler, which has '80s Shimano 600 calipers, and there was an improvement in braking over the DiaCompe levers they replaced. Another plus about the Tekro is the comfort of riding on the hoods. They fit my hands very well.