I'm not sure I agree. The diagonal separation of the brake system happens at the master cylinder.
You say no leaks- is the fluid level down at all? A master cylinder leaking back into the booster can destroy the vacuum diaphragm as well. Your hearing 'air leak' at m/c goes along with a vacuum leak from the booster. Does the engine surge when you apply the brakes at idle?

Loosen the master cylinder mounting nuts and ease it forward. There shouldn't be any fluid there.