It is the GMC shock absorber that helps control the up and down movement of your truck. This hydraulic suspension component makes it possible for a smoother ride for the occupants of the vehicle. A shock absorber is a self contained hydraulic system that cushions the ride by absorbing the movement of the truck when an obstacle is crossed while on the road. In time this suspension component on your GMC truck will wear out and be in need of replacing. The test for a GMC shock absorber can be done by the owner. By simply pushing down on one of the corners of the truck and releasing the pressure, the number of times the GMC truck moves up and down should be noted. If the movement is just up then back to a resting position the shock absorber is functioning properly. If you can count more than a single bounce up and then back down again, then the shock absorber is worn out and in need of replacing. This test should be done on all four corners of the vehicle. Another sign that this suspension component is in need of being replaced is when a visual inspection reveals a leak of the hydraulic fluid from the cylinder of the shock absorber. This occurs when the component is faulty from the factory or the load that the truck was carrying was too great and the seal on the shock has blown out. Either way the shock on the GMC truck should be replaced. If this suspension part is not replaced when it becomes worn, other components on the vehicle will be exposed to an increase level of vibration or travel which could cause them to fail over time.