I am performing maintenance on our aging Grohmix thermostat mixers part # 34 493 (we have two) due to wild swings in the water temperature when making small temp adjustments using the handle while showering. I cleaned the removed check valves (non-return valves part #14116) and found that the hot water valve internal spring loaded piston had failed (photo #1). The cold water valve was appeared operational (photo #2). I ordered replacements (part # 1411600M) and noted upon arrival that the new valve design lacked the internal spring loaded piston (photo #3). This puzzled me so I called Grohe and eventually spoke with someone who said that the valve design had changed and recommended that I use the two newer valves in one mixer and the two older valves in the other mixer while I await their sending two new replacement valves. I have performed this action and find that the mixer with two older valve works fine but the mixer with the two new valves continues to have the same issue of wild swings in water temperature with small attempts to adjust the temperature using the handle. This seems like a major valve design change (the removal of the internal spring loaded piston photo #4) and I now wonder if it is even compatible with the older mixer housing that I am attempting to use it in. Has anyone experienced this problem and have a work around? Thanks!