The gasket in the pumping mechanism broke and a replacement was nowhere to be found so we made our own using a 3D Printer and NinjaFlex filament.
This gasket is for the "Gardena Comfort Pressure Sprayers 3Litre (867)", more info here: http://www.gardena.com/int/garden-care-tools/pressure-sprayer/comfort-pressure-sprayers-3-l/