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The load sensing valve is a valve with two pistons and is connected to the braking system of a vehicle via a connecting rod. Its job is to adjust the amount of fluid flowing to the rear brakes, which it does by sensing how much weight has been loaded on to the vehicle.
As a vehicle is loaded, the weight of the load compresses the suspension and it’s this that indicates to the load sensing valve how much fluid should be distributed. The heavier the load, the more brake pressure is applied to the rear brakes, but the opposite is true when the vehicle is unloaded. This is in order to avoid locking up the rear axle during an emergency stop.
Issues with the load sensing valve tend to originate from the linkages seizing due to their position on the vehicle (near the back axle under the vehicle) attracting dirt and debris from the road. If this is the case, then lubricant such as WD40 can help. In the event of a seizure of the load sensing valve, you will notice that rear braking is insufficient. Valve seizure will result in an MOT failure.