1979 Mercedes Benz 300sd Brake Pedal resistance not consist

1979 MERCEDES BENZ 300SD
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Brakes problem
1979 Mercedes Benz 300sd Two Wheel Drive Automatic 229,78 miles

Sometimes brake pedal feels normal. Probably at least 95% of time.Intermittently pedal goes down to hard stop where feels as if no vacum assist. Pump the pedal and it returns to normal feeling.

How do I isolate to determine the problem?
Jul 7, 2009 at 3:49 PM
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DR LOOT
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It sounds like you got a bad master cylinder, this is how to check it, apply steady pressure, if the pedal goes all the way to the floor you have a bad master cylinder, let me know what happens.
Jul 25, 2009 at 4:05 PM
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Replaced Master cylinder and braking action seems so much better and normal. However, by applying pressure and holding the pedal recedes slowly. It almost makes me think the booster vacum diaphragm may have small leak also.

Please advise how I confirm whether diaphragm leaking.
Jul 25, 2009 at 4:31 PM
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DR LOOT
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you may be correct, the way to check that is remove the vacuum line going to the booster and hook up a vacuum pump and make sure it is holding pressure, also check all of the vacuum lines underneath the hood and make sure they are connected along with the rubber T that they are not cracked or rotted.
Jul 25, 2009 at 5:27 PM