Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Diesel Discussion

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 09-09-2009, 12:05 AM
Registered User
 
Join Date: Sep 2004
Location: San Jose
Posts: 63
Unhappy Brake Booster problem

Hi All,

I did a search and did not find out much information. I have a 84 300D that the brakes when out. I've had them go out before and it was a loose vacuum line. This time I am not so lucky.

I've plugged the two lines on the brake booster line, removed the connection at the boost and measured the vacuum of the vacuum pump. It read 23 inhg, which I believe is very good.

I connected the booster line again and I measured the vacuum at each of the two lines coming off the main line. I left one plugged and measured the other one. On these two lines the vacuum slowly crept up to about 16-18 inhg. When I hit the brakes, the vacuum would fall to 5 or 0. Would this indicate a bad brake booster??

I plugged both lines up again and let the vacuum build. I moved my car foward and the brakes worked only the first time I hit them and then not anymore. So, it must be leaking from the pump or the booster??
Anything else I can try?
rene

__________________
rene
1983 300D
Reply With Quote
  #2  
Old 09-09-2009, 05:04 AM
Wodnek's Avatar
Vintage Mercedes Junkie
 
Join Date: Jun 2004
Location: Southeast Wisconsin
Posts: 1,661
There is a rubber gasket between the master cylinder and the booster. make sure its good.
__________________
1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

Reply With Quote
  #3  
Old 09-09-2009, 09:14 AM
winmutt's Avatar
85 300D 4spd+tow+h4
 
Join Date: Apr 2006
Location: Atl Gawga
Posts: 9,346
Does the pedal go to the floor or is it just stiff? Check for fluid in the booster.
__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42
Reply With Quote
  #4  
Old 09-09-2009, 10:13 AM
Registered User
 
Join Date: Sep 2004
Location: San Jose
Posts: 63
The pedal does not go down to the floor. It is very stiff because it does not have the brake booster assistance.

I read on another post that I could try to pull a vacuum on the booster and see if it holds vacuum. I may be kind of tough to do so because there is a threaded fitting on the booster and It will be difficult to get a tight seal.

Does my symtoms sound like a pump or booster problem? Those boosters are expensive, I want make sure it is the booster.

rene
__________________
rene
1983 300D
Reply With Quote
  #5  
Old 09-09-2009, 10:19 AM
toomany MBZ's Avatar
Registered User
 
Join Date: Nov 2006
Location: central Va
Posts: 7,820
Quote:
Originally Posted by 356nen View Post
The pedal does not go down to the floor. It is very stiff because it does not have the brake booster assistance.

I read on another post that I could try to pull a vacuum on the booster and see if it holds vacuum. I may be kind of tough to do so because there is a threaded fitting on the booster and It will be difficult to get a tight seal.

Does my symtoms sound like a pump or booster problem? Those boosters are expensive, I want make sure it is the booster.

rene
I've seen a procedure, the fella did use an odd assortment of reducers to hook up to the booster, but was able after a hundred or so pumps on the mity vac to determine whether or not the booster was toast.
__________________
83 SD

84 CD
Reply With Quote
  #6  
Old 09-09-2009, 10:32 AM
Automch
 
Join Date: Aug 2006
Location: Anderson, SC
Posts: 93
You are experiencing a failed booster. I just had the same issue with my SD. Rebuilt Boosters will be priced between $600 and $150.00. Much against my principals I bought one from Checker (aka O'Riley) for $150 with a lifetime warranty. Not too bad to change, be sure to replace the "O" ring between the booster and master cylinder. Good Luck!
__________________
Retired
ASE Master Car / Light Truck
ASE Master Medium Heavy Truck
EVT Master Technician Ambulance
EVT Master Technician Fire Apparatus

"Without Fleet The Pride Doesn't Ride"

1983 300SD 609K + still going strong!
1970 Monte Carlo
1994 Ford F150
Reply With Quote
  #7  
Old 09-09-2009, 07:39 PM
Registered User
 
Join Date: Sep 2009
Location: Saranac Lake NY
Posts: 11
bad booster ?

I just bought a 79 300 TD . Stiff brake pedal . I searched for info also . I read one post that talked about checking the o-ring . I loosened the master cylinder and heard a vacuum seal being broken . Hmmmm , still vacuum in the booster . That confused me more .Tightened it back up and the brakes worked well for a short while . Then ... I read the brake booster section in the Haynes manual . They say that the vacuum is built up on both sides of the diaphram and it is broken on one side when the pedal is pressed . To test it they say to press the pedal down a few times while the engine is not running to break the vacuum then, while your foot is pressing down on the pedal start the engine and if the booster is working the pedal will be drawn down a short distance to the floor . I just tried this on mine and the pedal was drawn down .I ordered a new booster earlier today .That will be my $245.00 test .Anybody else have anything interesting to shed some light on this? I have feeling it is the booster . Spike.
P.S. Since this is vacuum related I must mention my door locks work but not after sitting for a while . I doubt this is related to my brake problem but being new to these cars and this type of vacuum set up it wouldn't surprise me .
Reply With Quote
  #8  
Old 09-09-2009, 09:20 PM
Stevo's Avatar
Registered User
 
Join Date: Apr 2002
Location: NW WA
Posts: 6,299
Quote:
Originally Posted by toomany MBZ View Post
I've seen a procedure, the fella did use an odd assortment of reducers to hook up to the booster, but was able after a hundred or so pumps on the mity vac to determine whether or not the booster was toast.
Couldn't you use one of the tapered rubber adapters? I must be missing something
__________________


1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"
Reply With Quote
  #9  
Old 09-10-2009, 02:12 AM
Registered User
 
Join Date: Sep 2004
Location: San Jose
Posts: 63
The rubber fittings on the vacuum pump are sometime difficult to seal tight on the hoses or fittings.

Spikediesel, thanks, I will try that test. From your test, the pedal went down, doesn't this indicate that the booster is working?
__________________
rene
1983 300D
Reply With Quote
  #10  
Old 09-10-2009, 10:59 AM
toomany MBZ's Avatar
Registered User
 
Join Date: Nov 2006
Location: central Va
Posts: 7,820
Quote:
Originally Posted by Stevo View Post
Couldn't you use one of the tapered rubber adapters? I must be missing something
I would think use whatever works.
I have not done this.
The fella used a half inch hose on the threaded nipple on the booster.
__________________
83 SD

84 CD
Reply With Quote
  #11  
Old 09-10-2009, 11:27 AM
Registered User
 
Join Date: Sep 2009
Location: Saranac Lake NY
Posts: 11
Yes, My pedal did get drawn to the floor . And this should indicate a properly working booster .. or maybe not . I think maybe they are assuming this will tell you if you diaphram is faulty but what if it is something else ? I'm wondering if the valve/ mechanism [whatever is in there ... I'm not sure] that breaks the vacuum on one side of the diaphram is faulty and not allowing air in[or out?] . And, even though it's building vacuum [apparently] maybe it's just a little more than it started with ...enough to let the pedal move . I'm going to go loosen the master cylinder again and see if I hear the suck of vacuum after the car has sat all night[ I have no idea what this will tell me ] . Before when I heard it I had just run the car .Update later . Spike .
My bet is still the booster being bad .

356nen , When you did your vacuum test did you increase the engine speed to see if the vacuum reading increased also ?

Last edited by spikesdiesel; 09-10-2009 at 11:35 AM.
Reply With Quote
  #12  
Old 09-10-2009, 04:45 PM
Registered User
 
Join Date: Sep 2004
Location: San Jose
Posts: 63
I did not increase engine spead to see if it would increase vacuum. I did not think of trying this.

This mornings I measured the VP pressure with both little lines off the big line plugged. My guage indicated that the VP had 23 mmhg.

So, I then disconnected the fitting at the booster and pulled vacuum on the booster using my hand vac. I wish I had and old booster line that I could have used the fitting to make a better adapter. All I could do was use the little rubber fittings from my hand vac kit. After pumping lots and lots of pumps, the booster got down to about 14 mmhg. I could not seem to get it lower. It did leak down very very slowly. I only had the patients to wait till it got to 10.

I hooked everthing back up and pump my breaks and they did not have that super firm feel. I turned on my car and what do you know, my brakes are working. I went for a long drive and my brakes were fine. After aobut 25 minutes of city driving they stopped working again. Hmm, intermiten problem?

I drove to my mercedes dealer to order the master cylinder/ booster seal and after about 10 minutes of driving the brakes work again. I drove all the way home 15 minutes and they continued to work??? What is going on? I parked my car and I will check later to see if the system leaked out.

Any thought of this and why it started to work again?
__________________
rene
1983 300D
Reply With Quote
  #13  
Old 09-11-2009, 09:20 PM
Registered User
 
Join Date: Sep 2009
Location: Saranac Lake NY
Posts: 11
Well, got my new booster and put it in today . Brakes work great . Not the easiest job getting those four mounting bolts off way up behind the pedal . The booster had fluid in it . Looked like used motor oil[black] , pretty sure it's just brake fluid that has been sloshing around in there for a long while .Found a note in the original maintenance book saying the master cylinder was replaced in 1993 . I'm assuming the diaphragm is good and the problem with it must be something else internal . Maybe I'll hack it apart to see if I can figure it out . I guess what can be learned from this is even if your booster can hold a vacuum it can still be bad . I would also assume that if it can hold a vacuum your o-ring isn't bad .
356nen , did you order an o-ring from mercedes ? I was wondering how much that costs ?? I used my old one it was in great shape . I hear you can get one at the hardware store and they work fine . Spike .
Reply With Quote
  #14  
Old 09-11-2009, 11:31 PM
Registered User
 
Join Date: Sep 2004
Location: San Jose
Posts: 63
Spike, I am glad you got your problem solved.

The gasket was actually a square "o-ring" seal. It cost me $3.25, the brought it in from the warehouse so it took a day.

This morning I went out to see if the booster or system leak up to atmosphere and it did not. I drove my car all day and it worked perfect??
I think I am still going to replace the booster since it seems like an intermiten problem. I have a newer, more comfortable car I can drive but I perfer driving my benz.
__________________
rene
1983 300D
Reply With Quote
  #15  
Old 09-12-2009, 09:41 AM
winmutt's Avatar
85 300D 4spd+tow+h4
 
Join Date: Apr 2006
Location: Atl Gawga
Posts: 9,346
Uh his problem is almost certainly not solved. More than likely his issue was the fluid not the booster. Fluid means you mbc is or was shot. Keep a CLOSE eye on you brake fluid.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42
Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 04:41 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2024 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page