You can adjust your brake pedal position slightly to add more pressure when the Hillbilly Brake is engaged. There is a nut on the clevis on the bottom/backside of the brake arm.
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Couple of points on the installation and adjustment of the Hillbilly Brake.
Even though I'm mechanically "ept", I'm gettin' up there in years and won't be doing any pole dancing or contortionist routines. Lining up the fender washer and the nut to get the bolt started is a chore. Took a few tries to get it right. Hint: before starting, put a little bit of superglue around the hole on the fender washer, place nut centered on washer hole. It'll now be easy to slide the washer and nut combination up into place to start the bolt. More easier!
I installed the brake and needed to stop nose up on a 20* incline. The brake wouldn't hold. I needed to increase the distance of the threaded rod that goes into the rubber bellows to master cylinder. This increases braking force with pedal locked. To do this, you only need break free the lock nut, at the forward end of the clevis, on the threaded rod that goes forward into the rubber bellows. Should be no need to detach the clevis (though I didn't figure that out until after I took it apart - sigh). Simply mark the threaded rod with a felt tip so you can count the turns, loosen locknut, then rotate threaded rod CW (viewed facing forward) to increase brake pressure with lock engaged, or CCW to decrease pressure. I extended it by 2 turns. Tested to make sure still relatively easy to engage HB, but haven't checked it on a hill yet.