Abstract: An automatically actuated hydraulically powered vibrator system for loosening the contents of a semi trailer dump body so that it will break away at its natural dumping angle. The semi trailer dump includes a hydraulic pump which delivers hydraulic fluid under pressure to extend a telescoping hydraulic cylinder to raise the dump body to a dumping position. An adjustable normally closed pressure relief valve is set to open when the pressure in the cylinder reaches a predetermined level corresponding to a dump body angle a few degrees less than the normal dumping angle for the commodity being dumped. When the valve opens, hydraulic fluid is pumped to a hydraulic motor which drives a vibrator which is bolted to the dump body. The vibrator loosens the material being dumped and prepares it to slide out of the body at its natural dumping angle.