Step 1, attempt to clean with brake cleaner, parts washer etc
Step 2, put in dishwasher and be shouted at by other half
Step 3, attempt to use chemicals in order to help restore
Step 4, after attempts 1 to 3 have failed, open bin and gently place reservoir into said receptacle
Step 5, open eBay and buy a new one
I am using my old brake fluid float sensor cap as the new one does not have this however for £30 plus postage it was a no brainer for me.
@eb88 how much was yours direct from the dealer