My girlfriend took her bike out for a spin and noticed the coaster brake wasn't working at all. Instead of braking it just back pedals, not like a free hub, but if you back pedal the bike actually moves backwards.
While I know quite a bit about racers/time-trial bikes, I have no idea how to fix this. Does anyone here know what I can check/do to fix this?
It may be a broken brake shoe, or possibly a broken clutch spring. This usually happens when the axle nuts are not tightened to the correct torque (30Nm), or the wheel is put back on without the non-turn washer.
Take it into a shop that has a Shimano dealer account. If you want to fix it yourself, they can order you the entire internal assembly (it comes in one pre-assembled piece, part #1 on this diagram). Make sure you emasure the length of the old axle, and order the same sized new one. With some minimal work (this link), you can remove the old assembly, pop in the new one, and get your girlfriend back to riding.
NOTE: The above video shows how to remove the coaster brake shoes. You are not trying to do that! Simply replace the entire unit.
If you'd rather not do the work yourself, a shop shouldn't charge too much for it, and the work will be warrantied as well. They can also check the hub for any other damage.
Another thing to consider is how, and how much, you are using that hub brake. If you're taking it down hills and riding the hub brake to slow down, you are generating a LOT of heat inside that complex and fragile hub, which will cause warping, melting, brittleness, fatigue, i.e "bad things", so take a good look at the parts your pulling out. It may be that you need to supplement the hub brake with a decent external brake to prevent this from happening in the future.
