Brother MFC-6490CW No Yellow

[The_Burg]

Dad bought a second hand one that hadn't been used for a good while.
After running through the cleaning process a great many times eventually all was working except the yellow.
Brother want £160 for a new head, which isn't viable.

Any ideas how to clear?
Mum thought of filling an old cart with solvent and running it through, which makes sense.
Apparently there is also a service menu which can be used to purge it.

Any ideas?