I was driving and the brake pedal suddenly became VERY stiff. Brakes still work ok, but they require a ton of effort applied to the brake to stop the car. I kept driving and noticed that while on the highway the brakes feel normal...then return to being super stiff at low engine speeds. Engine also will no longer shut off with the key, I have to use the stop lever.

I removed the check valve, and apparently there is supposed to be something inside this thing? Mine is empty and is just a hole. Does this look like a failed check valve?

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

Hmm... that looks like a connecting adapter to me. I have one like it. But I have petrol and a 5.6l. My check valve is further up the line at about 6 inches. Looks like a pancake. Made of plastic.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car

That's exactly what it is. You will need to recover the missing components (spring, plunger and retainer plate) from the inlet on the pump. The loose parts blocking air flow is the cause of the vacuum loss.

How do I do this? Do I take the front plate off of the pump?

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

No. The pieces should be in the passage right under the valve. They are to large to enter the pump. Use a magnet or a piece of wire to fish them out.

Ah, ok...thanks. If I get the pieces out and the pump starts working again, will I damage anything by running it without the check valve until I can find a replacement?

__________________
Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

No damage, but your brakes won't work properly..

Vacuum Pump Fitting
MB# 004 997 18 72


Fastlane: Vacuum Pump Check Valve
http://catalog.peachparts.com/item.wws?sku=W0133-1620263



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ASE Master Mechanic
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Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
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Dynamometer.
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Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

I tried fisning out whatever is in the inlet hole...but nothing is coming out, and when I look inside I can see nothing obstructing the opening. Using a magnet yields nothing as well...Is the next step opening up the vacuum pump? Can this be done without parts going everywhere? Or am I better off leaving it alone?

__________________
Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles

Maybe take a piece of safety wire and make a small loop (so you don't poke at the vacuum pump check valve). See if you can fish something out that way.

Pulling the front cover is easy enough. But, if you do, be sure to get a rebuild kit. It is easy enough to do. Especially if you are going to pull the front cover off. If you are pulling the cover, rebuilding the pump (minus the bearing) only adds about 10 minutes to pull the piston and install new o-rings and check valves.

This thread made me a little paranoid because I drive the car so much. I actually pulled my main vacuum line this morning, checked it for leaks (again) and inspected the check valve on top of the pump (again). It is all still in there... Whew...

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AJ

1985 300D (SOLD)