In that case, I have but one other suggestion which is to echo one you made earler, that they are from very old stock.

Others have more expertise in these matters - of that I am certain - but soft plastics have solvents in them that make them pliable and over time the solvents evaporate and they become brittle.

At least you still have the model in one piece![sm=thumbs_up.gif][sm=thumbs_up.gif]