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On a CLK320, we have a low brake pedal. Bled the brakes, thick pads, operated parking brake few times, still low. Maybe this car just has a low pedal?
 

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2004 CLK 240 Coupe
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This sounds like a 'servo' problem. i.e., the Brake Assist System (BAS) that your brake pedal is connected to. A non-return valve within the brake booster (A7/7) should maintain a high pressure in the line.

What happens if you 'pump' the pedal?
 

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2001 E55 AMG
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This sounds like a 'servo' problem. i.e., the Brake Assist System (BAS) that your brake pedal is connected to. A non-return valve within the brake booster (A7/7) should maintain a high pressure in the line.

What happens if you 'pump' the pedal?
Sorry to butt in,my pedal is very soft but goes hard when I give it one pump...I have an intermittent BAS/ESP light. On my friends diagnostics it said brake light switch and diaphragm on the booster (servo). The pedal symptoms are just like air but bleeding doesn't get any out,I'm going to use his power bleeder but I need to change rusty brake pipes first. (MOT due mid December)...Is this non-return valve you refer to this diaphragm? Part# A0015401517..Cheers.
 

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2004 CLK 240 Coupe
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11,377 Posts
Is this non-return valve you refer to this diaphragm? Part# A0015401517..Cheers.
No. The diaphragm is something else.

We discussed the booster on this thread earlier this year, and I believe that the non-return valve is integral (inside) the booster unit. The design is common to most MB models.

http://www.benzworld.org/forums/w140-s-class/1532000-loud-hissing-behind-brake-pedal-after.html

Checking my brake pedal today, I noted that with the engine off, the pedal sinks about an inch. When the engine starts (and the vacuum assist kicks in) the pedal becomes very firm, and the slack all but disappears.
 
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