Brake booster failed, should I also replace master cylinder?

My brake booster failed, it will not hold vacuum.
Should I also replace master cylinder while I'm in there?

Tempting, but if it was me I wouldn't. Only because of the pain of bleeding brakes. If you have no rusty brake lines, and the cylinder is not giving indication of giving up the ghost, and the fluid has been changed every couple of years, then I'd hold off.

Just remember if you get air in the line ahead of the ABS module (such as replacing the master cylinder). You should have a service bleed done to ensure all air is purged from the ABS system, which requires a computer capable of telling the ABS to bleed itself.