My son's bike has a SRAM i3 internal gear + coaster brake hub. It has recently started sometimes only activating the coaster brake after backpedalling a good one or two full revolutions. When in this situation, it sounds somewhat clunky or grindy when the brake finally activates. However, sometimes, it kicks in straight away. I haven't found a solid pattern to when it works and doesn't, but it seems that the brake is more reliable in gear 1 than in gear 3, though I might be wrong about this. Can this adjusted/fixed, or do I need to replace the mechanism?
EDIT: After experimenting with the brakes and gearing with the bike in a stand (previously I had been trying while riding it) it seems that in gear 1, the brake always works fine. In gear 3, it requires almost a full rotation backwards before the gear engages. While in gear 2, it's somewhere between - about half a rotation.