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Old 06-25-2006, 11:02 AM
mespe's Avatar
Join Date: Feb 2005
Location: Cleveland, Ohio
Posts: 1,848
No Power assist to brakes when car first started

HI all,

I've seen this on a couple of 300SD. You start the car, let it idle for 10-20 seconds. When you press on the brake, a very increased pressure is required, as if you have no vacuum boost. Is this normal because vacuum is slow to build up, or do I have a problem somewhere? If this was normal, I'm sure the engineers at MB would have made it un-normal by adding a vacuum storage device. Both cars I've seen this on would shut off fine.

Anyone got an idea?


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Old 06-25-2006, 11:13 AM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Do you wait the 20 seconds before pressing on the brake, or is the test done immediately upon startup?

I'd be fairly sure that the booster requires 15-20 seconds at idle to completely evacuate. In fact, I hear some of the vacuum doors operate after about 15 seconds or so on the SD.

But, any time outside of this window might indicate a weak vacuum pump.

Pretty simple to check. Put a vacuum gauge on the main line and start it up.............
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Old 06-25-2006, 12:23 PM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 35,252
i would have said bad check valve on booster line.

i am not sure though.

but i would think that normally the booster will maintain vacuum if all is right with the system.

tom w
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

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Old 06-25-2006, 12:35 PM
Join Date: May 2002
Location: Blue Point, NY
Posts: 25,396
Originally Posted by t walgamuth
i would have said bad check valve on booster line.

i am not sure though.

but i would think that normally the booster will maintain vacuum if all is right with the system.

tom w
That's a real possiblity. I've got to check that valve on the SD as well.
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Old 06-25-2006, 01:19 PM
Registered User
Join Date: Sep 2004
Posts: 8,731
Assuming that the vacuum system itself is okay, there are three major possibilities:
Failed check valve in the booster line.
Poor seal at the master cylinder/booster interface.
Leaking booster, which will require replacement.
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Old 06-25-2006, 02:26 PM
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Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
I think the design paremeter by most manufactures is enough vaccum retention in the booster for at least one power brake application if engine say quits on the road. If my thinking is rational and system in good leakproof condition that retained vaccum should be there when you first start your car. Then required vaccum operational build time is also reduced signifigantly if so. As the vaccum pump becomes perhaps older and less efficient everything would still appear normal. You would just be topping off the system at worse. Unfortunatly in the real world there are variences to put it mildly. Or another way to look at it. Mercedes had no optional charge for all the potentially active but unplanned future bleed off points in their choice of vaccum circuit components and materials.
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Old 06-25-2006, 02:58 PM
Hatterasguy's Avatar
Join Date: Nov 2002
Location: Milford, CT
Posts: 19,306
Vacuum is stored right in front of your left foot under the front fender. You get a few vacuum assited brake pedal pushes when the car is off. After that its all manual.

Unless the car has been sitting for months you should have brake assist before you turn the key.
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Old 08-27-2008, 10:25 PM
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Location: New Cumberland, PA
Posts: 833
Digging up an old thread here...
When the car starts, the pedal is very stiff and not very effective (I'm guessing no boost assist). After a few seconds (I haven't timed it, it could be 10-20) braking is back to normal. This has happened twice in the past week, but not the other five days that the car has been started in that week's time.

Is it safe to assume that the booster is fine but I might have a leaky check valve? Is there a way to check the booster for leaks? I suppose I can try checking the check valve with a mityvac and some creative fittings. I'd much rather replace the check valve.

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