If it is anything like the tractors I drove years ago, it is a front brake limiting device, designed to sharply limit the power of the steering axle's brakes when travelling empty or under conditions of poor traction such as snow and ice, to keep the front wheels from breaking traction so they can be used to control a braking skid.
Some tractors were sold into the 1970's without brakes on the steering axle, which made for, shall we say, "interesting" handling characteristics when travelling bobtail (without a trailer) on ice.
