Maybe I'm setting these brakes up wrong but the springs are not strong enough to center the brake. They are Avid 20 canti's. There are three holes in the mount and I am putting them on the position for most tension. Do the centering springs on Canti's wear out? Can they be bent to recover their old spring?

Two things...

1) Often there is a small allen bolt that you can use to increase/balance/fine-tune the brakes. The bolt is at the rear of the brake near the canti boss. Most Avid brakes have this, although I don't know if your model does. Turn it inward to increase tension.

2) Yes, the springs can be manually "bent" to increase tension. It's a lot easier to do with linear springs (more common on v-brakes), but possible with coil springs too.

Are you talking about Center Pull Cantilever brakes or Direct Pull V Cantilever Brakes such as normally found on comfort and MTB?

The latter, there is a screw near the brake pad on each side and this adjusts the tension to pull the brake off the rims. It's common to have one brake pad riding closer to the rim than the other side.