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Old 01-06-2002, 11:27 AM
Dave F
Posts: n/a
vacuum pump question

I need a little help on diagnosing a brake problem on an 83, 300SD. At times, the brake pedal becomes hard and makes the car hard to stop. I really need to push on the brake pedal to stop.

I put a vacuum gauge on the vacuum circuit and I get a reading of 15 inches of hg. on the gauge. I inserted my vacuum gauge by removing a vacuum line and replacing it with the gauge. If I increase the rpm I cannot get a reading over 16 inches of hg. The manual for job 43-0023 indicates that I should have .7 to .8 bar of vacuum. I have 2/3 if that.

The car shuts down when the key is turned off, the ACC flappers appear OK and the transmission shifts OK. I get no vacuum loss in the vacuum lines at all. Any thoughts on further diagnosis?

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Old 01-06-2002, 12:48 PM
Senior Member
Join Date: Jul 2001
Location: San Antone
Posts: 408

Sounds like you have a problem with a partial vacuum pump diaphragm/piston failure, the main vacuum line leaks or the fitting is loose where it connects to the vacuum pump or brake booster, the inline surge dampner on the main vacuum line from the vacuum pump is leaking or the plastic is cracked/broken, or the brake booster diaphragm is damaged (these would be the areas I would look at on my '77 300D W123 anyway).

When you tapped into the vacuum system to check vacuum which line did you tap into? Best place is the main vacuum line running from the vacuum pump to the brake booster that has a small vacuum line running from the T-fitting that provides vacuum to the rest of the system. After looking for obvious problems (cracks, breaks, loose connections, etc. - you may need to clean the components to inspect them with a flashlight and mirror to view the underside & hard to reach areas) and if none are found then disconnect the main vacuum line at the brake booster, then plug the T-fitting line, and see how much vacuum from the pump you get with nothing connected to the pump other than the main vacuum line. Then check the brake booster to see if it holds vacuum and how much it holds & for how long (a very slow, minor vacuum loss over time may be OK - I do not recall the official M-B spec for vacuum loss over time, but there is a spec). If your vacuum tester fittings will not allow you to connect directly to the vacuum line connection on the brake booster, then you could carefully pull the large main vacuum line apart at the T-fitting to connect the test gauge there. Then check the inline surge dampner that is located close to the T-fitting on the main vacuum line.

If you do not find any problems, then the vacuum leak could be in another part of the vacuum system (AC, door locks, trunk lock, fuel door lock, etc.). These other vacuum systems have to be checked as a whole to find the sub-system that is the problem, then each component and vacuum line within that sub-system to locate the problem (been there, done that - it is time consuming).

Some other members will own/have experience with your specific model and can give you some other area/components to check. I am using what my car's vacuum system has that is in common with yours (both are the same/similar regarding the pump, main vacuum line, brake booster, T-fitting, etc.) - which should help you begin with diagnosing your problem.

Good Luck!
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Old 01-07-2002, 02:27 PM
Registered User
Join Date: Jan 2000
Location: PA
Posts: 5,440
I would connect the vacuum gage to the main line from the vacuum pump to the brake booster and run the line inside the car so you could see what the vacuum is when the brakes don't work correctly. You should be able to determine if it is the booster or the vacuum pump that way.

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Old 01-10-2002, 10:36 PM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150

Most likely you have either a leaking booster or a bad vacuum pump diaphram.

Check the vent line from the vacuum pump to the intake mainfold for oil -- if there is any significant oil in the line, the vacuum pump diaphram is shot.

How much vacuum do you have with the engine shut off? Can you apply the brakes three or four times before you lose boost, or is the pedal hard immediately? If you have no reserve vacuum in the booster, it is probably shot. I had one on my old Dodge Aries that leaked so badly the engine would stall when I applied the brakes!

If you have reserve vacuum, but never have enough in the line, check for leaks -- the lines get hard and loose on the fittings. If you don't find any, you probably need a new diaphram in the vacuum pump. This isn't a big deal.

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 01-11-2002, 06:17 PM
Dave F
Posts: n/a
Thanks for the replies. I found out the vacuum pump was indeed shot. I had 14" of vacuum where I should have at least 20" of vacuum. I replaced it and I now have 20".

I took the old one apart. There is no diaphragm on the pump. There are two springs, the roller bearing and the piston. My guess is that the piston was leaking.

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Old 01-15-2002, 06:52 AM
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Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
Most of the time, for the piston type, the check valves in the pump fail. Pumps used on older 617 engines (possibly 1980 or older) are diaphragm type. Both diaphragm and piston types (vacuum pump for 603 engine cannot be rebuilt) can be rebuilt . Rebuilt kits are available. If you have a rebuilt kit, the check valves can be replaced without removing the pump, just the cover.


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