This picture shows why these interior castings can be difficult to remove. The interior of the insulator often contains irregularities like bubbles and annealing lines created by the original glass molding process. Since these become filled with the mold making material when attempting a pinhole casting, they serve to "lock in" the casting. In this case, you can see the raised lines caused by the all-to-common shallow cracks (annealing lines?) found in EC&M pinholes. Other older insulators commonly exhibit similar features that could cause difficulties. Newer stuff like Whitall Tatums would probably be less of a problem.

For this reason, I would strongly suggest using a flexible material for pinhole castings!