Firm Brake Pedal

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.

that's exactly what i mean. how can i adjust that?

You will need a repair manual for your car model because each car is different. On my Dodge, the linkage from the brake pedal to the master cyl. has an adjusting nut that can be adjusted to take the slackness out of the linkage. Beware, you do need some freeplay (manual will state the distance of the freeplay) otherwise you will have a very touchy brake that will tend to lock up at the slightest pressure on the pedal. For example when you want to slow down to keep distance from the car ahead of you on a highway, that freeplay helps otherwise the moment you tap the brake, it will apply max. braking pressure which you may not want.