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Diagnosing Power Brake Problems

On my 76 Lincoln Mark IV, I’ve recently had a problem develop where after using the brakes a couple of times while driving, or being stopped for an extended period (e.g. at a stop light), the brake pedal goes soft. If I press the brake pedal further, I can then stop the car. Is this something wrong with the brake booster or the master cylinder?

Very well could be. If you are not going to fix it yourself why is it not being towed to a brake shop. If you are going to try to fix it yourself I wonder if you should seeing as how you are asking for help. Brakes are not something to mess with and get wrong.

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That’s a classic indication of a brake master cylinder leaking internally.

When the car is cold, the brakes work fine.

But when the under-hood temperature increases, the brake pedal starts sinking to the floor when sitting at a stop.



Your symptoms point me in the direction of the brake master cylinder

Not surprising considering your car’s age

I’d get your car on a lift . . . or at the very least, 4 good jack stands . . . and inspect all of the brake lines and hoses, to make sure that you don’t have hydraulic problems beyond just the master cylinder. Your brake hoses might be in horrible shape, not saying they are, but it wouldn’t surprise me, considering your car’s age


Why would different temperatures make the master cylinder behave differently.?

The master cylinder has rubber seals inside it to build pressure. Rubber expands with heat quite a bit more than the cast iron (or aluminum) of the bore. Worn rubber seals that expand, may not expand evenly since they didn’t wear evenly because of geometry and gravity.

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I got a new master cylinder on and she stops like a dream! Thanks everyone!


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