If when you press the brake pedal it feels too firm, then check the following:

Vacuum issues

It's possible that there is a vacuum leak in the brake booster system. 

Check the booster system for any leaks. 

Check the vacuum hose from the engine to the booster, check for kinks or damage. 

If not, it is possible that the brake booster is faulty. Replace if necessary.

Brake line obstruction 

It is possible for something to block brake fluid from reaching a portion of the system. 

It could range from a chunk of rust or a pinched brake line. Inspect and replace if necessary.

Brake pedal linkage

Check the pedal linkage to see if the linkage to the booster is correctly adjusted. 

It is possible that linkage has been over adjusted where the pin in the booster is pushing up against the master cylinder making the system brake even when the foot is off the brake pedal. Adjust if necessary.