w115 brake issue - Mercedes-Benz Forum

 
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post #1 of 7 (permalink) Old 08-26-2011, 05:40 AM Thread Starter
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w115 brake issue

So I was cruising in my 76 300d w115 the other day and once it got up to running temp (~180), the brake warning indicator came on and it seemed like i lost brake boost...brake pedal would go almost all the way to the floor before lightly breaking....

fortunately i had just arrived to where i needed to be....

got back in the next day and everything was fine...until she got warm again...

any thoughts on where to look?

ian
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post #2 of 7 (permalink) Old 08-26-2011, 08:05 AM
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I'd look in the neighborhood of the vacuum pump.

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post #3 of 7 (permalink) Old 08-26-2011, 11:54 AM Thread Starter
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Hey - i don't have a vacuum gauge - but i pulled off the vacuum supply line to the brake booster and it was sucking enough air to hold it against my finger pretty well.....and that wouldnt explain the light staying on...

The brake fluid level is right where it should be....and when braking really hard there is no pull to either side...as a pressure difference can also cause the light to stay on...right?

or is a pressure difference between the front and back brakes? how to troubleshoot?!

thanks!
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post #4 of 7 (permalink) Old 08-26-2011, 11:28 PM
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Sounds like the booster has gone out on you. Although, how long since you've gotten the brake fluid changed and the system bled?

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post #5 of 7 (permalink) Old 08-26-2011, 11:50 PM
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Quote:
Originally Posted by RadioTek View Post
Sounds like the booster has gone out on you.
If the booster went out the pedal wold be extremely hard to push, not easy.



The master cyl has gone out on you. To test this theory, pump the pedal 50 times, I bet before you get to 50, it will act the same as if it was warm. No brake pressure.

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post #6 of 7 (permalink) Old 08-27-2011, 10:36 AM Thread Starter
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Does anyone out there know what would happen if i replaced the MC that is on there (this one has the electronic sensor) with one that does not have the sensor?

Both models were apparently used in 76 300D's....but the one without the sensor is practically free - while the one with the sensor is 200$+....
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post #7 of 7 (permalink) Old 08-28-2011, 07:31 PM
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Yours with the sensor is what triggered the early warning red light. The early style without the sensor will (cannt) not give you a warning. No other difference.
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