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1983 300SD, 4 speed manual, 1994 C280
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Discussion Starter · #1 ·
Been bleeding brakes after rebuilding all 4 caliphers and replacing the Master Cylinder. Now when I am pumping brakes up I can hear a hissing noise from the vac pump up front. I don't know how long that has been going on but I have noticed a slight run on when I turn ignition off (2 to 3 seconds). Also I have noticed a slight flair in shifting every now and then. I am wondering if the diaphragm has/is getting a tear in it. Does anyone have any experience with the diaphram in the pump going out? Can that be changed out pretty easy? Thanks for the help!
 

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1983 300SD, 4 speed manual, 1994 C280
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229 Posts
Discussion Starter · #2 ·
bumppp....
 

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Always Remembered RIP
2015 ML 400 x 2
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If you don't get an answer, try sending jdc1244 a PM, he is one of the all time diesel guru's.
 

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300SD
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I believe that the later models (81-85) have a piston type vacuum pump. No diaphragm in that one. You can test the vacuum pump easily with a vacuum gauge.

Disconnect all branch vacuum lines from the main lines, and cover the nipples with rubber vacuum line caps (available at Auto Zone for a bout $2 a set). Then start the engine and connect uyour vacuum gauge (Mity Vac pump with a gauge will do).
If the vacuum pump is good, you should see about 21 inches of vacuum .

You could also have a leaky power brake booster which DOES have a diaphragm. The hissing could be coming form it.
 

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1983 300SD, 4 speed manual, 1994 C280
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229 Posts
Discussion Starter · #5 ·
I rebuilt the vac pump and that hiss went away. Thanks for the replies.
 
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