The brakes on my 240D have been erratic recently, so this weekend I replaced the worn rotors and brake pads, flushed and replaced the brake fluid, and bled the brake lines. However, when I test-drove my car to check the brake function, I discovered that I had lost all vacuum assist from the brake vacuum booster. The brakes function, but require considerable effort before engaging.

I can also report that the fluid level in the front of the master cylinder reservoir was low, although an inspection indicated no leaks at the calipers, and that the fluid level had dropped since I checked it 300 miles ago.

I don't believe that my vacuum pump is defective, since my locks continue to work, and the engine still shuts off. Thus after consulting my manuals, I assume either the vacuum booster or the check valve is at fault. I've tested the booster per the manual--pumping and holding the brakes and starting the engine--and there is no movement in the pedal when the engine starts.

Are there other tests I can do before ordering a new booster? Is a used booster from Atlanta Stuttgart Auto Parts an option, or should I order a new one? Can anyone suggest a way to remove the check valve from the line coming from the vacuum pump? So far it has resisted even my most insistent efforts to remove the valve for testing.

Does anyone have any words of advice before I proceed? Thanks.

Ken Mitchell


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You still could have engine oil getting past the vacuum pump and/or the diaphragm in the shut off fastened onto the back of the injection pump. Get two of the small air filters from your dealer (actually, I think they are fuel filters also). Install one in the line coming off the shutoff and the other in the line where the vacuum system branches off from the main line between the vacuum pump and the brake booster. If you have oil coming from either source, this will catch it and identify the source. If the check vaslve remains suspect, squirt carburetor cleaner through it, then blow it out. Often clears things up. Brake booster rebuild kits are available and not too difficulty to work with.

Ken,

I expect that the problem lies in the master cylinder. If you were replacing the cylinder it would require "bench bleeding". I expect that somehow the system is not bled correctly.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Really about all you can test is whether the booster gets vacuum. We have a special adapter to measure the vacuum after the check valve (NOT REMOVABLE) at the booster. During proper application there will be around 20 in of vacuum which will drop about 5 in when the pedal is depressed and very quickly return to original vacuum. There should be no further loss of vacuum as the brake pedal is held. It should expell vacuum each pedal stroke, but the level shouldn't drop below 10-15 in on one stroke. Every booster can be pump "dry" so to speak by rapid pedal application.

Without being able to test after the check valve, I would remove the line from the booster and place your thumb over it and run the motor. Check the vacuum while teed into the source (leave all the rest of the systems alone) nipple on the line. Remove your thumb and the vacuum should drop. Replace it and it should reach full vacuum in seconds.

These boosters are very trouble free so used can be a reasonable option, but their wholesale price is so low that you might want to check around before going to the labor of used. The greatest percentage of bad boosters are those that have sat open for great periods of time - such as in junk yards.