That looks a lot like a Load Sensing Valve, they are commonly used in utes and light to heavy trucks to increase braking to the rear wheels when there is additional load in the rear. However, when there is no load, the braking force is not required to be as strong so the valve reduces the braking pressure to the rears and prevent spinning. Due to the car being lowered and the axle being closer to the chassis, the valve thinks there is a greater load and would be applying more pressure. I would suggest you have it professionally adjusted as they are often very sensitive and if you mess it up it could have serious consequences.