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 12-18-2010, 09:17 PM
1986 300sdl 1985 380se
 
Join Date: Aug 2005
Location: West Monroe LA
Posts: 484
86 300sdl Brake pedal sinking at idle

Good evening all,
a new perplexing problem with my 86 300sdl. First of all I have new rotors, brake pads, sensors, and rubber brake hoses. Fluid level in the master cylinder reservoir is normal and not changing. Brakes work excellent EXCEPT occasionally at idle at a red light my pedal will sink slowly to the floor almost.
At first I thought master cylinder, but no leakage and no fluid level drop.
In my driveway with the car off, I can pump the brake pedal two or three times to release all of the built up vacuum in the booster and the pedal sits there hard as a rock. So I am thinking not the master cylinder.
Today at a red light it did it again so I pumped the brake pedal 4 or 5 times and the pedal comes all the way back to normal, but I hear a whooshing sound at my feet. Could my booster be leaking? Could I have a vacuum leak somewhere? My transmission shifts perfectly so I was ruling out the vacuum source for it, BUT... a Climate control vacuum leak? Could that affect your brakes?

Thanks in Advance for all comments.

__________________
John
1986 300sdl
1985 380se
Reply With Quote
  #2  
Old 12-18-2010, 09:20 PM
vstech's Avatar
DD MOD, HVAC,MCP,Mac,GMAC
 
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,835
ok.
if you come to a stop, and hold your foot lightly on the brake, and the pedal drops to the floor, your seals in the MC are shot and you need to either rebuild yours or put a new/rebuilt one in.
period.
sometimes they will leak internally and not lose any fluid.
sometimes they will leak into the booster, and you will lose fluid.
__________________
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!
Reply With Quote
  #3  
Old 12-18-2010, 09:26 PM
1986 300sdl 1985 380se
 
Join Date: Aug 2005
Location: West Monroe LA
Posts: 484
Quote:
Originally Posted by vstech View Post
ok.
if you come to a stop, and hold your foot lightly on the brake, and the pedal drops to the floor, your seals in the MC are shot and you need to either rebuild yours or put a new/rebuilt one in.
period.
sometimes they will leak internally and not lose any fluid.
sometimes they will leak into the booster, and you will lose fluid.
You know, my common sense told me that, but I was just hoping it wasn't the master cylinder. Time to buy a new one. I have read that you must bench bleed a master cylinder prior to installation,. I have never done this before. Is there a kit you can purchase?
Thanks for the quick reply.
__________________
John
1986 300sdl
1985 380se
Reply With Quote
  #4  
Old 12-18-2010, 09:30 PM
babymog's Avatar
Loose Cannon - No Balls
 
Join Date: Jan 2006
Location: Northeast Indiana
Posts: 10,765
What he said.

If it is sinking, step down harder. It often will stop sinking if it is a bad master cylinder.

Sometimes a brake job will cause this failure, some feel that it is due to pumping the brake pedal further than the normal travel to bleed, which can damage the seal as it crosses a wear-ridge in the bore.

Yep, new MC time.
__________________

Gone to the dark side

- Jeff
Reply With Quote
  #5  
Old 12-18-2010, 09:30 PM
vstech's Avatar
DD MOD, HVAC,MCP,Mac,GMAC
 
Join Date: Mar 2006
Location: Mount Holly, NC
Posts: 26,835
most MC"S come with a bench bleeder kit.
for some reason, the MB MC's don't. I think if you put a rag under your mc, install it, with the lines loose, then fill the reservoirs, they will bleed themselves. when clear fluid weeps out the line nuts, tighten them, then proceed to bleed the brakes at each wheel normally.
__________________
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!
Reply With Quote
  #6  
Old 12-18-2010, 09:43 PM
1986 300sdl 1985 380se
 
Join Date: Aug 2005
Location: West Monroe LA
Posts: 484
Quote:
Originally Posted by vstech View Post
most MC"S come with a bench bleeder kit.
for some reason, the MB MC's don't. I think if you put a rag under your mc, install it, with the lines loose, then fill the reservoirs, they will bleed themselves. when clear fluid weeps out the line nuts, tighten them, then proceed to bleed the brakes at each wheel normally.
Awesome! That is the way I did it on my old Toyota Corolla ( 1974 ) way back when I was in college. Believe it or not that was the last master cylinder I ever had to install. This MC in my 86 is the original one with 24 years and 122,000 miles on it. I guess I have had good service out of it. The MC in my 85 380se with 257,000 miles on it is the original. I am a firm believer in yearly brake fluid changes. The sdl does not get driven as much and this probably had an affect on my calipers and brake hoses which I just replaced.
My old brake hoses were almost swollen shut.

Thanks for the input and advice!
__________________
John
1986 300sdl
1985 380se
Reply With Quote
  #7  
Old 12-18-2010, 09:43 PM
layback40's Avatar
Not Banned
 
Join Date: Nov 2009
Location: Victoria Australia - down under!!
Posts: 4,023
Quote:
Originally Posted by vstech View Post
most MC"S come with a bench bleeder kit.
for some reason, the MB MC's don't. I think if you put a rag under your mc, install it, with the lines loose, then fill the reservoirs, they will bleed themselves. when clear fluid weeps out the line nuts, tighten them, then proceed to bleed the brakes at each wheel normally.
& if you are real careful when you disconnect the lines from the old MC, when you put the new one in, if the MC is bled you can get away with not bleeding the whole system. its worked for me.
You do need to change out your BF regularly though, if you havent done that in a while, best do the change.
__________________
Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving
Reply With Quote
  #8  
Old 12-18-2010, 10:20 PM
Registered User
 
Join Date: Sep 2001
Location: central Texas
Posts: 17,281
1.Power brake booster seal is bad.
2. Napa parts sells a metric bleeding kit ( reusable ) .. about $10 when I got one last.
3. Do not try to install a MC which calls for bench bleeding without bench bleeding. I have that that in my younger days and the work taking it back out to do it correctly is totally wasted energy.

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 08:39 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