How to tell the difference between air being in the brake lines or needing to replace the master cylinder?

I have to push further down to stop my car. It s not that bad but I m taking the car in Monday. I d rather be safe then sorry. When the car is stopped I pump the brakes and it stiffens but then if I push hard the pedal still goes down. It kinda feels like the pedal doesn t stiffen until 2/5 of the way down. There is still plenty of break fluid so it doesn t look like a leak… Model: 2004 Ford Escape.

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