That's the actuator / sensing spring to your Load Sensing Proportional Valve (LSPV).
When breaking, the truck sinks in in the front and goes up in the back.
The going up in the back causes to be less downforce (sometimes called 'load') on the rear axle. This may cause the brakes to lock the wheels. The back then starts sliding and gets uncontrollable.
This mechanism senses how much the back goes up when braking, as one end is attached to the axle and the other end to the LSPV on the frame.
The spring actuates the LSPV, which proportionally sends back some brake force from the back to the front and thus balances the braking force and prevents the rear wheels from locking.
Although called 'load' sensing proportional valve, it has little to do with the payload you put on your truck.
Cheers Ralf
