I've just taken the cheek pads out of my Astro. It's a couple of years old and starting to get a bit ripe inside. I ended up removing the cheek pads. (just give them a but of a tug and they will come.) The rest of the lining however id glued in so I ran a deep warm but not hot bath full of water and just submerged the whole lid. Amazing how much dirt there was inside and amazing how fresh and clean it now smells!
It did take about 4 days to dry out fully though!