|You need stronger king pins. Back when I used sure grip, I had Titanium king pins. They lasted forever and they always came out with ease.
The problem is that you cannot find them anymore. So, try to find some other extra hardened king pin. Preferably, one made for skateboard trucks. Something that is meant to do grinds on. Roller skate king pins, especially Sure Grip, tend to be made of very lightly forged metal, and susceptible to stress related problems.