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Q: Brakes are failing no fluid leaks at all and brake fluid resevoir still full

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Ok so it was fine the day before but next morning i jumped in and when i went to use the brakes the pedal would slowly sink to the floor was a tad scary but was able to come to a slow stop, there is absolutly no leaks and the brake fluid reservoir is still full, the fluid is also a normal colour and clean, after taking it around the block a few times its still full with no leaks anywhere that are visable but the pedal is still sinking to the floor and im feeling like it will fail completely soon and i wont be able to stop at all, i have stopped driving it of course as i do love my life and im sure other road users do to lol so of course my main question is whats going on could it be as simple as buying a new master cylinder also the brake pads are fine? And if i was to drive it and it was the master cylinder could i loose all break control completely? Thank you very much in advance for your answer.

My car has 200000 miles.
My car has an automatic transmission.

An internal seal failure in the master cylinder is possible and that is easily tested for by either blocking off the master cylinder ports and/or using pressure gauges to measure pressure developed within the system upon pedal application. It is possible that the problem is due to present air in the system or a leak that has admitted air on repeated application of the brakes over time. Of course, the system can be thoroughly bled to rule that possibility in or out. If the wheel bearings are loose, that will cause excessive pedal travel. Also, it is possible that a clogged proportioning valve could play a role in this. To answer your question about safety of operation, you are right that in this particular scenario, it is manifestly unsafe and unwise to operate the vehicle until it is repaired. If you desire that this problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request a low brake pedal diagnostic and the responding mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.

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