One of the most important and serious issues is linkage slop. The pivot on top of the axle often has a wallowed out woodruff Key. Look carefully at the rearward facing lever, and the front handle and move it through the range. There cannot be any lost motion there. Mine was moving 4 degrees before moving the lever on the PTO. If yours is loose fix it first. A new bigger Key, and a broached keway, plus dremel tool work to fix the key pocket can make them tight. This will allow stronger engagement and function.
Adjustment: A Band type with the adjusting screws under a small dome, first make sure there is no lost motion in the linkage where it pivots. Move the lever to half way in its travel and bungee it there. Loosen both bands so the shaft turns by hand. tighten the front band so the shaft won't turn by hand, then loosen it about 1-1/4 turns, or till the shaft just turns by hand, then the same with the back band. This allows the drive band to be fully released when braking, or the brake band fully released when driving. Jim