Abstract: An injection piston for a die casting machine comprising, a piston die having a chamber therein open at one end, a die rod having an end slidable in the chamber for changing the volume thereof. A coolant is supplied to the chamber to act as a damper to the movement of the die rod toward the press die when the injection piston is moved under pressure into a mold. The coolant flows in a coolant path comprising a feed path defined in the die rod and a discharge path at least a portion of which is defined between the press die and the die rod and the rest of which is defined in the die rod. A slidably mounted immersion valve is in the end of the die rod adjacent the chamber and movable into a neutral, a first and a second position for selectively permitting the flow of coolant into and out of the chamber, blocking the flow of coolant altogether and permitting the flow of fluid only out of the chamber depending on the increase of pressure exerted on the injection piston as it is forced into a mold.