Small bits on the bolts that hold the brake pads.
I found that running fuel tubing on the linkage gave a more progressive feel to the brakes. With the spring, they brake at almost a constant level throughout the servo movement.
Without anything, brakes feel like a light switch. On or OFF only.
With fuel line, the more you move the trigger, the harder it puts on the binders, so it feels as if you have better control to slow the truck down instead of just locking up the brakes or having too soft of brakes.