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?

Marty

www.benzbonz.biz

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.............

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

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..I also have a 427 Cobra replica with an aluminum chassis.

That's a real possiblity. I've got to check that valve on the SD as well.

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.

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.