Abstract: A nonreturn piston slide valve has an inner housing operatively held in an outer housing and a spring operated piston operatively held in the inner housing for cooperation with a valve seat. A flow passage is formed between the piston face and the valve seat when fluid flows through the valve in the intended direction. When the fluid flow direction is reversed, the valve closes. In order to prevent so-called flutter movements of the piston the piston side wall is provided with a number of apertures through which the inner volume of the inner valve housing may communicate with the fluid flow. The apertures move with the piston to facilitate rapid opening of the valve during a first portion of a valve opening piston stroke. During a second end portion of a valve opening piston stroke the apertures pass into the inner housing thereby braking or dampening the movement of the piston at the end of the stroke to prevent slamming.