Hydraulic pressure is equally applied to both shoes. However, forward rotation wedges the shoes tighter to the drum. When going backwards, the rotation tends to push the shoes away from the drum. This effect is used on boat trailers with surge (drum) brakes that have a trailer brake master cylinder in the trailer coupler, so you can back the trailer even though reverse motion is applying the trailer brakes. If a boat trailer has disk brakes, there has to be a solenoid to release hydraulic pressure back into the trailer master cylinder to back a disk brake boat trailer.
brakes don't stop well when rolling backwards
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