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I have to backpedal in order to brake. But when I do this, it works for about 3 seconds and then there's some kind of noise and my pedals start going on their own. The momentum of my back tire would push my pedals to keep going and I couldn't stop them with my feet.

I let my friend try riding it so he could see what it was doing and when he tried to backpedal, the seat came off.

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    The trouble with internal coaster brakes is that when they go wrong, there's no easy fix without taking them apart, which is rarely economically viable unless you have the tools and skills yourself. Having said that, there will be an 'arm' attaching your hub to the frame of the bike; this stops the whole hub moving (turning) under braking. Check to see that this is tightly secured. – JHCL Oct 19 '15 at 9:14
  • Yes, I would first check that the strap that secures the "arm" is in place and properly wrapped around the tube of the frame, so the "arm" can't move. – Daniel R Hicks Oct 19 '15 at 11:44
  • Even if you have the tools, you need to have the know-how for the particular model of brake (which basically requires it being in the Sutherland's Handbook), so beyond making sure the arm is tight, you're best off leaving it to a shop if you're attached to this bike. You do need to have the arm down tight though. – Batman Oct 19 '15 at 16:09
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    Noone else mentioned it, but I think your seat problems are unrelated. Could be a good idea to do a bolt check on the whole bike and make sure nothing else is going to come loose under you. – Criggie Oct 28 '15 at 4:37
  • Have you had any results ? Curious to know what the progress is. – Criggie Nov 2 '15 at 22:46
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What I'm guessing is happening is that your reaction arm (the thing on the left of the diagram) is not fixed in place.

When you brake the shoes of the brake are pushed out from the axle into the shell of the wheel hub. The idea being that the axle is fixed in place and the shoes drag against the shell.

Cutaway diagram of a coaster brake

If the reaction arm was undone, the friction of the brake shoes against the hub shell would be greater than the friction of the axle in the dropouts and so the whole unit of hub, axle and cog (fixed to axle) would turn under the rotational momentum of the wheel.

My guess is the 3 seconds followed by a noise is the time it takes for the reaction arm of your hub to overcome the friction and force its way past the chain stay.

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