Have you concidered 'Non embedding lapping media' I had a problem with some valve gear once (I actually made and scrapped 4 sets - about 6 months work) untill I came accross this stuff . I now use Ajax scouring powder (usual disclaimer) and refine it by letting it sediment in water but you will...
If I may suggest:-
1 Make certain that the axle of the drive wheel is in line with the radius through the centre pivot.
2 Make certain that the castors have some 'Lead' on them so that the wheels follow their pivots.
3 Make certain that drive wheel is not too heavily loaded
4 get the drive wheel...
