[herr_oberst]

There's a spring tension screw on the brakes, it's a tiny black screw, I can see it in the picture. One on each arm. Be careful with that screw, it's JIS, not phillips and easy to cam-out. But I'd also recommend removing the brake arms and greasing the bosses. While those brake arms are off, you might see three holes where the spring tails grip the frame; you can increase spring tension there, too. And if you're feeling adventurous and froggy you could take apart and lube up the cable, but pay heed, it's easy to fray the end of a cable and not be able to get it back into the housing.

Edit, get a piece of 100 grit sandpaper and give the rubber brake pads a swipe; they've surely hardened by now. And scotchbrite the wheel rims.