Replaced brake pads on car today due to a shake, would typically diagnose to warped rotors but they were recently replaced and tested to be balanced. replaced pads and drove around lot to settle pads. pressure remained consistent but on the way home I tested a higher speed stop. 35-40mph and brake pedal depressed fully to the floor and pressure did not immediately return. no drop in fluid level and in no leaks on cardboard beneath car.
My car has 242000 miles.
My car has an automatic transmission.
When the brake pads are worn out and replaced by thicker pads, excess brake fluid in the lines will push back into the master cylinder when the caliper is depressed. In some rare instances, this may cause air bubbles to be trapped in the lines; which can cause the pedal issue you're experiencing. The first thing I'd do is bleed your brake to see if the pedal issue is fixed. If this doesn't help, contact a professional mechanic to have them complete a brake system inspection so they can trace the source of this issue as soon as possible.
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