You've lost leverage with the shorter pedal. You can get more mechanical advantage by using a shorter arm on the pedal where the actuating rod attaches - ie locate the actuating rog closer to the pivot, or a longer arm on the brake cam lever - locate the actuating arm attachment farther from the pivot... or both. This may make the pedal travel too long though, it's pretty much trial and error and changing the attachment points isn't too easy.
If you've had the rear axle loose, try loosening it again and then tightening it while someone stands on the brake pedal. This centres the brake plate in the drum and can improve braking quite a bit if the thing has been tightened off centre. There isn't much play on the axle but I've seen a real difference doing this.
