Brass in aluminium — BAD idea! The brass, (in fact the copper in the brass), and aluminium are too far apart on the galvanic scale and the aluminium, being the more reactive, will be the one to suffer.
Make the tube out of wrought aluminium or stainless steel it’ll be a lot safer and kinder to your cylinder head and maybe preserve the last of your threads.
Dissimular metals + water – a whole area of research in its own right!