1. Agree with Freddy, you need a gravity or pressure feeder. Also, are the hoses routed so any part loops above the level of the bleed screws on the caliper? If so, you might have an air pocket there.
2. Although the brakes should be self-adjusting, you might need to shim the pads on one side to equalize the gap.
3. Finally, if the spindles are slightly bent/tweeked from contact or temporarily operating the karts as a lawn mower :) this can result in the caliper being misaligned with the rotor. Generally you can shim around that.