Q: Brake pedal sinks

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Brake pedal sinks, I added fluid, it wasn't very low.brakes was good before. It keeps taking fluid. I haven't seen any signs of leaks at calipers, master cylinder, or under dash. Pedal will feel good when sitting if I pump it. Once key is turned it sinks. And yes my check engine soon light comes on shortly after driving. Could this be the master cylinder?

My car has 82000 miles.
My car has an automatic transmission.

This is a sign of a potential vacuum leak in the brake booster. When there is not enough vacuum in the brake booster, this will cause a hydraulic pressure drop resulting in a soft brake pedal. If you’d like, a certified YourMechanic technician can come to your location and help you with a Brake pedal is lower than usual Inspection to confirm that this is the issue, and also help to perform a Brake Booster Vacuum Pump Replacement to repair this issue.

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