A dispenser is provided for dispensing the extrudable contents of a collapsible container (10) having a nozzle (22) through which the container can be discharged by collapsing the container (10). The dispenser is of a piston/cylinder device, with the piston (70) having a container-engaging face provided with a contoured structure of a predetermined configuration corresponding generally to a collapsing profile of the collapsible container (10). An elongated flexible valve device (128) is slidably mounted relative to the housing and includes a coupling end coupled directly to a trigger member (78) of the dispenser and a valve end (140) for opening and closing an opening in the housing which defines the cylinder of the device. The flexible valve device (128) is a band member of plastic material and is operatively associated with a mechanism for holding the valve open when a cover of the housing is removed and for automatically closing the valve when the cover is replaced. A pistol grip handle (98) and the trigger (78) of the dispenser are ergodynamically configured for ease of operation. A set of interchangeable stop cams (156) are provided for positioning between the trigger (78) and the pistol grip handle (98) for selectively determining a metered amount of material to be dispensed by the dispenser for each actuation of the trigger (78).