Dear Marc1974ch and GP_SYSTEMS,
you are trying to do some magic on the K8400. However, in my opinion your proposed ways are suffering some drawbacks.
First of all it is nearly impossible to print flexible filaments with the original extruder housing. Within this housing the filament is bended, an when you push a bended rubber, this part will bend more. In this way the filament is treated too, and when it is bended in the extruder housing, it becomes completely blocked.
The first step is to use an improved extruder housing design. I don’ checked the design proposed by GP_Systems, for myself I’m using a similar reference: http://www.thingiverse.com/thing:1133248. For this design I am sure that the filament is guided straight through the extruder housing.
In this case there is only a physical limit for the filament pushing: the critical compression force. If you have a bar and press the two ends together, the bar will bend when a critical force is achieved. The same happens to the elastic filament. Depending on the elasticity of the filament this force will be lower for more elastic materials. So we have to take care to hold the pushing force of the extruder for the filament below this critical value.
Unfortunately the pushing force is determining the extrusion rate, so with more elastic materials the maximum extrusion rate is reduced. Consequently you have to reduce the extrusion rate when you are processing flexible materials. In my opinion the loosen of the bearing pressure will introduce a certain slipping in the pulley, which have the same effect. The drawback of this solution is the reduction of printed material. I believe, that the prints are not that impressive like with “standard” filament.
I have printed for myself flexible material with a TPE of 50 (Flex-1000 TPE 50). To avoid a blocking of the filament I had to reduce the extrusion speed to 10mm/second.