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 > Vintage Mercedes Forum

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 07-26-2020, 12:57 PM
Registered User
 
Join Date: Jul 2020
Posts: 39
Hard Brake Pedal 1967 200D

Hello

I have a 1967 200D that recently started having a hard brake pedal problem. I have to push very hard on the pedal to get the brakes to work. I suspect something is up with the brake booster or vacuum system. I can hear a sucking sound from inside the cab when i press on the brakes, but don't hear anything under the hood (engine noise may be masking it). Any advice on where to start troubleshooting this is greatly appreciated.

Thanks

Reply With Quote
  #2  
Old 07-26-2020, 10:51 PM
Diesel911's Avatar
Registered User
 
Join Date: Sep 2007
Location: Long Beach,CA
Posts: 41,428
Quote:
Originally Posted by bhammontree View Post
Hello

I have a 1967 200D that recently started having a hard brake pedal problem. I have to push very hard on the pedal to get the brakes to work. I suspect something is up with the brake booster or vacuum system. I can hear a sucking sound from inside the cab when i press on the brakes, but don't hear anything under the hood (engine noise may be masking it). Any advice on where to start troubleshooting this is greatly appreciated.

Thanks
You have already targeted the 2 main areas. Check the vacuum first.

On my newer model you need 22-24 inches of Hg of vacuum to get the power brake booster to function properly.
If like 15 inches of Hg or so of vacuum it could be the check valve in the vacuum pump is shot. Otherwise the innards of the vacuum pump have issues.

Someone else will have to say how to troubleshoot the power brake booster as I have not done it for over 40 years.

If you are in the US many auto parts stores will loan you a Vacuum or a combination vacuum and fuel pump gauge. So you don't need to buy one. However, it is up to you how to figure out how hook it up.
__________________
84 300D, 82 Volvo 244Gl Diesel
Reply With Quote
  #3  
Old 07-28-2020, 12:26 AM
Registered User
 
Join Date: Jul 2020
Posts: 39
I got a chance to check vacuum tonight. It was reading 22 in Hg at the pump and 21 at the booster inlet. I went ahead and replaced all the old vacuum hoses just in case, but it didn't help the brake issue. I also noticed the car is smoking more than usual since this started. Planning to replace the oring seal between the master cylinder and the booster tomorrow night and see if that helps.

If it does turn out to be a bad booster does anyone know where I can get one or rebuild this one?. I saw a few in Europe for over $1000. Not wanting to spend that mich, so any ideas on rebuild or retrofit another less expensive booster would be much appreciated.
Reply With Quote
  #4  
Old 07-28-2020, 01:41 AM
Registered User
 
Join Date: Sep 2001
Posts: 1,819
White Post Restoration, White Post VA does boosters and masters. I've no idea what it costs.

Karps Brake Service, Upland CA.
__________________
Does a Caddy stretch? No, but a Mercedes bendz.
Reply With Quote
  #5  
Old 07-29-2020, 01:11 AM
KCM KCM is offline
Registered User
 
Join Date: Jan 2004
Posts: 641
Had the same problem a couple years ago. If you check the vacuum while pressing the pedal you will likely see the vacuum drop way down due to the valving in the booster. Mine had leaked brake fluid into the booster over the years and likely contributed to diaphragm deterioration. Luckily I bought a new booster on Ebay many, many years ago for little money. Really never thought I would need it but turns out I did. Not much for brakes when the vacuum pump or booster fails. FYI, the booster part number is 000 430 75 30.
Reply With Quote
  #6  
Old 07-29-2020, 01:47 PM
Registered User
 
Join Date: Oct 2012
Posts: 17,375
When I was in my teens I had a Ford with power brakes. This was very rare during those years and few mechanics outside of the dealers knew how to service them.

But I noticed the car starting to smoke and also a drop in performance that I knew had to be coming from the automatic transmission. Those were also rather rare so I had two things on this car that were new and strange to most auto repair shops.

But one guy I knew was always taking on new things because he figured he needed to know how to fix cars since that was his business. So I left it with him for about a month while he played with it.

What he found was the brake booster had developed a pinhole in the diaphragm that was allowing the engine vacuum to pull brake fluid out of the brake tank and burn it with the gasoline. Yeah, it was plumbed up a bit different from what we are used to today.

But the pinhole was so small the booster would hold vacuum for a bit then slowly start to fall. He noticed his vac pump was showing a few drops of what smelled like brake fluid so he put two and two together and came up with....

Replacing the brake booster, the master cylinder and installing a check valve in the vac line from the intake manifold to the brake booster.

Now with yours being a diesel I can't understand how brake fluid can get from your vac pump to the intake so perhaps the smoking in your case is another matter having nothing to do with the brakes.

But when pulling a vacuum it should not just pull down but also hold for hours. Otherwise you have a leak somewhere in the vac system. It could be the insides, it could be the metal flanges... It could be the body of the booster itself.

First I would try a good used one off of Ebay. Make sure the seller swears they have vac tested it aud it holds for at least one hour.
Reply With Quote
  #7  
Old 07-29-2020, 10:29 PM
Registered User
 
Join Date: Jul 2020
Posts: 39
I replaced the o-ring between the master cylinder and the booster tonight and no improvement. I am thinking it must be a bad booster, so I am looking for a new or used one. If anyone has one they would like to sell please let me know. Also, if anyone has any experience with retrofitting a more common booster onto this car I would love to hear from you.
Reply With Quote
  #8  
Old 07-30-2020, 09:36 AM
t walgamuth's Avatar
dieselarchitect
 
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 37,825
I suspect you could eliminate the booster and run a smaller master cylinder. It might work pretty well even without going smaller on the non-boosted master.

I had a 52 Pontiac on which I simply eliminated the booster and ran unboosted with no problem.
__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
Reply With Quote
  #9  
Old 07-31-2020, 09:11 AM
Registered User
 
Join Date: Jul 2020
Posts: 39
I decided to check the vacuum pump again before replacing the booster, and now I think it may be a problem with the pump. The first time I checked the pump it pulled a steady 22 In Hg, but this time the needle on the gauge was bouncing around violently around 5-10 In Hg. I pulled the pump and the diaphragm looked OK, but wondering if it could be a bad check valve? This pump has the internal check valves. Does anyone know if I could replace the internal check valves with external check valves, or where I could get replacement internal checks?
Reply With Quote
  #10  
Old 08-03-2020, 10:59 PM
Registered User
 
Join Date: Jul 2020
Posts: 39
So I cleaned the pump up and put it back together and it now seems to be working properly with 25 In Hg. Unfortunately the brakes are still not back to normal, though they do seem to have a little more power than before. I can hear a hissing sound when I press the brakes that last about 1 second. I removed the booster tonight and noticed a small tear in the rubber boot around the pin that attached to the brake pedal. Would a tear at this location lead to vacuum leak? Would it be worth trying to repair the boot with shoe glue or something? Also I am striking out on trying to find an exact replacement booster so if anyone knows if other years or models are able to be retrofitted in this car please let me know.

Thanks!

[IMG]
Reply With Quote
  #11  
Old 08-04-2020, 07:43 AM
t walgamuth's Avatar
dieselarchitect
 
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 37,825
Sounds like that boot is a dust cover and unrelated to performance.
__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
Reply With Quote
  #12  
Old 08-04-2020, 07:11 PM
KCM KCM is offline
Registered User
 
Join Date: Jan 2004
Posts: 641
I agree that tear is just the dust cover and will not cause a leak. Do you have a MityVac or other vacuum pump and gauge you can connect to the booster vacuum port? I would test the booster by itself first. May take quite a few pumps with a MityVac but the booster should build vacuum and hold it for a while. This would definitely tell you if it is the booster.

When you tested the vacuum pump, did you put a tee in the line to the booster or disconnect the booster and put the gauge directly into the line going to the pump? If you check it without the tee first to verify the pump is working, then put the tee in and you don't have enough vacuum, that would also tell you the booster or seal between the booster and master cylinder is leaking.
Reply With Quote
  #13  
Old 08-04-2020, 10:56 PM
Registered User
 
Join Date: Jul 2020
Posts: 39
I took the master off of the booster again tonight and cleaned up the mating surface and replaced the o-ring again to be sure that seal was good. I then bench tested it with a hand vacuum pump. I was able to get it to about 10 In Hg with the hand pump. I then put it back on the vac line from the cars vacuum pump with a tee in the line and was able to get it up to 23 In Hg. As soon as I pushed in the brake pedal plunger in it goes to 0 In Hg and I hear a hissing sound for about 1 sec, then climbs back up to 23 In Hg over the course of 30 sec or so. When I turn the car off it slowly bleeds down to zero over 8-10 minutes. Does this sound like a bad booster, or something else? I haven't put it back on the car yet to see how the brakes are working, but plan to do that tomorrow night.

Thanks
Reply With Quote
  #14  
Old 08-06-2020, 09:19 PM
Registered User
 
Join Date: Jul 2020
Posts: 39
I think it is has a hole in the membrane. It will hold vacuum fine until I hit the brake, then it goes to 0. I found a booster off a 1966 250 SE on Ebay that looked almost identical so I bought it and am hoping for the best. It came with the master cylinder as well. Hope to be back on the road in a few days.
Reply With Quote
  #15  
Old 08-07-2020, 01:18 PM
KCM KCM is offline
Registered User
 
Join Date: Jan 2004
Posts: 641
Sound like a bad diaphragm. I believe there is an internal valve that closes off the vacuum until the pedal is pushed. Would explain why full vacuum when the pedal is not pushed and zero when it is. Plus the booster should not hiss when the pedal is pushed.

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 10:29 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2022, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2018 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page