And a better view of how the brake actuator works! First, tap a flatblade screwdriver under the flanged retainer on the pivot post, and pry it off. Leave the one on the cam alone, the replacement brake pads only come with one replacement retainer! Pull the actuator arm 90 degrees to force the pads apart. Jam appropriately sized junk from the parts bin between the pads to hold them apart. They won't clear the flanged retainer on the cam just yet, so now stick something that won't mar the cam surface between the actuator cam and a pad (I used a scrap of insulated electrical wire) and repeat. This time the pads will be further apart and will clear the retainer on the cam. Prop them open and lever them off of the pivot post with the nearest flatblade screwdriver. I don't have a good trick for reassembly. You'll nearly have it, then the spring will fall out. Repeat until you accidentally succeed.

The back of the chassis in the temporary head cab. The bias test points are the red and black knobs off the back of the chassis, just right of center. The bias pot is accessed through a hole in the top of the chassis to the left of the power tubes. The pot shaft sticks partway up through the hole and is adjusted with a flat head screwdriver, and is insulated from the chassis with a rubber grommet. I'll make a back for this cab in a day or two, but right now I want a bit of a break from the project.