I need help with my Epson 7600 large format printer, the magenta colour isn't working?!?

I've been given a Epson Pro 7600 large format printer - which hasn't been used in over 3 years and the printer has only ever print about 45 mtrs of mainly black acetate.

I've brought new cartridge for the machine, done the usually cleans and one power clean - which has cleared all the heads prefectly except one - the magenta.
I've had a look at the lines and all the lines which i can see that run along the bottom of the printer and over the top show good ink flow, no air bubbles or anything.
But the magenta is show a big air bubble (i assume it's an air bubble as it is totally free of any drop of ink, and is very clear) about 4cm long right at the begining of the line - the bit that goes into the 'damper'/'carriage' - I don't really know what this is called.

Can anyone help with info about how i can sort this? I don't really want to pay ££££ to call out a technician (plus i don't think there are any in my area) - especially if someone knows how to clear this.

I've tried cleaning and cleaning and cleaning the heads in the hope is blows it through.

Has anyone come across this or knows how i can sort it? My husband is quite good at dismantling etc if need be.