The best approach to soften the brass is to heat the brass to a temperature that removes prior cold work. This temperature is called the annealing (aka recrystallization) temperature.
Depending on the amount of prior deformation to the brass, the annealing temperature can vary from 350 deg F up to 1100 deg F. You may need to experiment on optimizing the annealing temperature.
Does brass have to be quenched quickly to remain soft at RT?
Other copper alloys?
(I ask cause this si what was reccommended to me when replacing my dad's 1971 Triumph (T100C) head, the gasket was to be heated to bright red adn then quenched in water, then lapped on each side on a very flat lapping stone (old surface plate). I've also been told that this can be used to restore banjo-bolt copper washers to soft state for re-use. (note: because the cost of new washers and gaskets is minimal compared to the cost of failure we used new ones.))
