Depends on what you like for firmness. First you have to make sure there are no leaks and air has been bled. Normally a spongy brake where the pedal sinks even after the car has stopped indicates a leak or air in the system.
If you mean that your brake pedal has some free play before it engages, that can be adjusted and also the return spring can be replaced if that has worn out.
Don't spend the money on a master cyl. until you have checked out all other possibilities. I have had cars over 20 years old with perfectly fine working master cylinders.