Rog, you could try this, but i think yours is fuckin YEARS of build up in there lol. It is deffinatly being held back being caked in there like that.
The shit i was thinking of today was the Caustic soda deal, the same as drain unblocker/oven cleaner. Jam a potato or wine cork in the end of the pipe (stinger end) and then pour it in. Give it a good dose then add HOT water a bit at a time till it's almost full then plug the cyl end off if ya want. Give it a good shake as you go. This method saves having to mix up corrosive stuff and then try to pour it in without spilling it on anything you like, such as your own hand. You will need to do it a few times to get it clean. Leave it a while between times to work. Remember it is corrosive so don't leave it for a week, hose it out and then leave it hanging in the garage. Wear GOOD gloves and a mask if you have one or just hold your breath .
Then dump the shit out into a metal bucket and dispose of properly.
Or you could also use an map gas torch and burn it out...but that will probably mess the chrome up and could be dangerous if not done right.