If you need the Hillbilly brake to hold with more pressure, there's a simple adjustment you can make. On the front of the brake pedal arm, there's a clevis, locknut, and threaded rod going forward to the brake cylinder. No need to undo the clevis, just loosen the locknut with wrench, then turn the threaded rod a couple of turns clockwise (looking from back towards front). Tighten locknut. Test.
A quick hint for installing the Hillbilly brake. It's a pain to get the fender washer and serrated nut all lined up with the bolt. Much easier if you first put a small amount of super glue on the fender washer and glue the nut to it. Then you can slide the "assembly" into place with one finger.
