Greg,
-Ever noticed how some colours are more prone to breaking than others?
-On Neve consoles it's always the light blue coloured caps which crack/snap.
Here's my best tip:
Get a plastic knife -like those handed out free at the delicatessen- and file down a thin -but not too sharp- edge on it. Take the sharpness off with some steel wool.
That should be able to sneak in under the cap, and you can either 'roll' it around a stationary cap, or "bow" it along the edge of a slowly rotating cap.
What happens is that the plastics aren't totally stable over a long time period, and they weaksn and become soft, brittle and crumbly.
Something in the dyes seems to exacerbate this in SSL green knob caps and Neve light blue knob caps...
Keith