Q: Brakes don't work properly

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We have replaced the master cylinder and bleed all the brakes and when the car is off the brakes are firm but when the car is running the petal is soft and it goes to the floor and don't come back up like it don't have any back pressure
My car has an automatic transmission.

If your vehicle is equipped with anti-lock braking, you have to either bleed the ABS pump and valving (per Factory Service Manual instructions) or use a bi-directional scan tool to remotely actuate the ABS system during brake bleeding. Otherwise, in an ABS equipped vehicle air will remain trapped in that segment thus causing a low brake pedal. If you do not have an ABS system, the only explanation for what you are describing is trapped air, an internal leak within the master cylinder (of course look for external leaks, but you have probably done that already), or possibly bulging rubber brake lines. If you desire that the brakes be bled by a certified Mechanic, dispatched by YourMechanic right to your location, please request a brake system inspection/diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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