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Brake Pedal Sinks when hot

On one hot day (90 degrees) this week after driving on the highway for about 30 miles and then sitting at a light my brake pedal sunk about 1 - 2 inches. It did this again when IO stopped to park. This is an 2001 PT Cruiser with a standard transmission and 44k miles. The original brake, pads , rotors and calipers are apparently in good shape. I have not driven it under the same circumstances again and the problem has not re-appeared. Some additional background, this car has anti-lock brakes and requires a special diagnostic tool to release the abs system to bleed the brakes. I generally bleed my brakes every few years. A few weeks ago I decided to merely siphon off a few ounces of fluid and replace it with new Dot 3 fluid every week for about a month. I used certified Dot 3 fluid, but purchased an Off Brand name. Also, I?m getting ready for a longer (1000 mile) trip south where it will be even hotter than 90 degrees. Any thoughts?

A sinking brake pedal is a sign that the brake master cylinder is leaking internally and requires replacement.

Tester

Yup. You need a new brake master cylinder.

Brake master cylinder is bad. In a panic stop…you may not.

Reminds me of an old car I bought from a friend, as a fun fixer-upper. He said, “The brakes work. You just have to pump 'em.”

Yes, the master cylinder was the first part I replaced. Other brake parts, and tires, in short order after that.

You may have moisture in your brake lines. If the pedal is only spongy in hot weather after hard braking, brake line moisture may be turning to steam … giving you a “rubber” pedal.