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  #1  
Old 06-23-2012, 08:22 PM
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New Brake Booster and vacuum pump is good... What is going on!

I have just replaced the brake booster just knowing that was what was keeping the 1985 300SD from shutting off and a hard brake pedal. Well no, and I took the line loose next to the vacuum pump and checked it with the mity-mite and it held good vacuum. Anyone got some help. Bumfussled.

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1985 300SD
1990 300 2.5 Turbo
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  #2  
Old 06-23-2012, 08:34 PM
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Can you hook the mightyvac up to see what kind of negative pressure you have at idle?
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  #3  
Old 06-23-2012, 08:47 PM
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Well Joe, I really can't tell from your brief write up just what you are doing, but take the simple approach. You have the vacuum gauge right. Well here is "the plan". On the big black line that runs from the vacuum pump to the power brake unit, you probably have two, maybe three small line that "T" off that big line. Now get your mighty vac gauge hooked up to one of those small hoses, just your gauge, nothing else. Then any other "T" on that line, disconnect it and plug it off. Golf tee will usually plug them off easy. Now that is less then 5 minutes work. Now start the engine and see how much vacuum you are pulling. Write it down. Now shut the car off. You won't have vacuum there so use the red shut off lever by the pump. Is it holding its vacuum? How much? How long will it hold it? Can you see the gauge moving? Write it down and give us your report. We can probably help you.
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Last edited by junqueyardjim; 06-23-2012 at 08:52 PM. Reason: more info
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  #4  
Old 06-23-2012, 08:53 PM
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Sounds like another frustrating troubleshooting by replacing Parts job.
Getting and using a Vacuum Gauge or Mightyvac type Vacuum Tester to find the problem is cheaper.

Where the Vacuum Tubing screws into the Vacuum Pump is a Check Valve. When I had hard braking and shutoff issues removing the Check Valve and hosing it out with WD-40 cured the problem. (I am not sure if Brake Cleaner will damage the Plastic Parts inside so I did not use that to clean it.)
But, some have removed the Check Valve to find that the innards were broken.
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  #5  
Old 06-23-2012, 09:13 PM
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the "vacuum pump is good" line scares me due to my pump just imploding and taking out my valve cover and stopping the motor last week... on my 85SD with only 214K on the clock...

please measure and report back the results of the above test so we know where to direct you.

you do know there is an o-ring on the back of the master cylinder that seals the booster right?
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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  #6  
Old 06-24-2012, 10:00 AM
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New Brake Booster and vacuum pump is good... What is going on! Rev. 2

Thanks to the gentlemen that responded to my problem I got up this morning, first made sure the check valve was not stuck, all good there and very dry. I used the brown shut down vacuum line off of main black hose next to brake booster. Hooked up the Mity-mite to it, removed the 4way vacuum hookup while running plugged that line both ways had zero vacuum. hooked back up while running and turned ignition off, hooked mity-mite up to brown line and pulled vacuum and shut engine down.
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  #7  
Old 06-24-2012, 10:48 AM
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Sounds like the vacuum pump is inoperative or there is something plugged up.
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  #8  
Old 06-24-2012, 11:21 AM
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I should at the very least be reading some vacuum, would you agree Engatwork?
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1985 300SD
1990 300 2.5 Turbo
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  #9  
Old 06-24-2012, 11:46 AM
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It's not entirely clear to me from your description where you are reading the vacuum, but yes you should be reading vacuum if you are on a line hooked to the main line with no interference.
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  #10  
Old 06-24-2012, 12:30 PM
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OK,
do this.
disconnect all small vacuum hoses from the large main vacuum hose, and plug them
disconnect the large hose from the brake booster, and push the mite in the end of the hose and start the car. what does the mite read?
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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  #11  
Old 06-24-2012, 01:08 PM
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You should have vacuum being provided by the vacuum pump if the engine is running.
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  #12  
Old 07-15-2012, 02:45 PM
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I am sorry for such a long delay in what has gone on with my 300sd. I bought a new vacuum pump and installed yesterday and this fixed all my problems. Thanks to everyone who gave input. This site is invaluable.

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1985 300SD
1990 300 2.5 Turbo
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