Q: Brake sinks slowly while at stop

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96 Honda Accord EX V6. Brake pedal sinks slowly all the way to floor while at a complete stop. Usually takes about 10-15 seconds for Pedal to be completely at floor, with absolutely no pressure and car starts moving. When this occurs, I can pump brake pedal which takes pedal slightly back up, but once I leave constant pressure again, sinking starts again. There are FEW times I've driven the car WITHOUT this occurring, but cannot conclude what factors were present at the time (i.e.; outside temp, car haven't been driven in a few days). Except that it almost always happens when A/C is on vs off. More info: NO problems with brakes when depressing while driving, no longer than usual stopping distance, not spongy otherwise. Checked brake fluid (normal). And NO brake fluid leaks found undercar at all. Braking power is perfectly fine while normally depressing pedal as you would when coming to a stop. Older car with unusually low mileage. What could this be?

My car has 970000 miles.
My car has an automatic transmission.

The most common cause for the brake pedal to sink slowly to the floor will be the brake master cylinder internal pressure cup failure. The seals may be worn or sludged up from old dirty brake fluid.

I have seen where brake pads were replaced and when the caliper is pushed back with the bleeder valve closed will push dirty brake fluid back into the brake master cylinder and then when the brake pedal is pumped up the debris get into the master cylinder rubber piston cups causing them to not seal any more.

The fix would be to flush out the entire brake system with fresh brake fluid [https://www.yourmechanic.com/services/brake-system-flush] then replace the brake master cylinder with a new one and bleed and flush the system out completely. A certified professional from YourMechanic can come to your car’s location to inspect the sinking brakes so that this gets repaired correctly.

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