Abstract: A safety device protects a mud pump from an over pressure condition, which can occur during normal operation. The mud pump discharge couples to a high pressure inlet at the bottom of a valve body and the discharge outlet from the valve discharges to the side of the body, ninety degrees to the inlet. In a first aspect, the safety device includes a load piston mounted to a shaft which passes through a pressure chamber cap and connects to a pneumatic cylinder assembly on the top side of the pressure chamber cap. The pneumatic cylinder assembly is operated by a control valve mounted to the base of the cylinder to set the pressure at which the safety device actuates. In a further aspect, the safety device includes a light transmissive indicator cylinder, with a flag therein mounted on a rod coupling the valve to a pneumatic actuator piston, between the actuator piston and the valve.