the brake calipers are "floating" calipers meaning they have the ability to float back and forth small amounts to allow for slight irregularities in the rotor and for brake pad wear. They float back and forth on lubricated pins that mount the caliper. There are rubber boots that are supposed to protect these pins from rust and corrosion. I find that these boots do not work that well and allow the pins to get rusty and corroded. Once this happens, the caliper cannot float anymore and is stuck in a fixed position. If this fixed position is such that the pads are in constant contact with the rotor, this explains your problem. I have had this problem with every caliper on my trikes. Fixing this can be tricky depending on how ceased the pins are inside the caliper. Sometimes it takes a little heat and penetrating oil to get them loose. The good news is that once they are out and apart, both the replacement boots and pins are still available from honda. Good luck. I bet that is your problem.
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