In school one of the instructors showed us what he used... I made one and still have it 40 years later. Mine is made from 1/8" weld filler rod, is probably 9-10 inches long, and is shaped like a comma. One end is nearly straight, the other has a loop (curled with needle nose pliers) so it doesn't dig into your palm.
Start by poking this tool into the backing plate slot, find the star wheel then probe just inboard (toward the center of the axle) and you will feel the spring-loaded resistance of the adjuster lever. Press it away from the starwheel, then steady the tool between your first finger and thumb right against the backing plate. Insert the adjusting tool with your free hand and loosen the wheel up.
SC
Is there a special tool for this? I have
the "brake tool" but that's what you turn the starwheel with.