What cement did you use ? With Testors cement you just got yourself a new cement brush, its really hard to get that out again. If it was Tenax7R just dip the brush in the jar until it softens up and wipe it on a rag, repeat until all of the plastic residue is gone and then clean with paint thinner. The stuff that goops up your brush after using Tenax is not the glue, its dried, liquified plastic from the last time you used the brush, I have a brush specially for Tenax but in a pinch I could clean it up to paint with again.