I had to recently flush out about 100 bearings to change the grease to another type for a test. We contacted the manufacturer and they recommended we place the bearings in a hydrocarbon (diesel fuel) that is heated. This will dissolve the old grease and clean everything.
We did it in stages, with 3 heated containers. The bearings went in the first container (a cup) and put in the oven for about 15 minutes. They were then shaken. This did the bulk of the cleaning. Then we transfered the bearings to the 2nd cup which had clean diesel fuel, and repeated. Finally, we transfered the bearings to the 3rd cup. All 3 cups were heated. This did a great job cleaning the bearings and getting them ready for re-greasing.