Brake Booster, master cylinder or Vacuum Pump? 300sd - Page 2 - PeachParts Mercedes-Benz Forum

david s poole · #16 10-05-2006, 04:27 PM

ok enough tests.find a booster and put it in the car.the good news is------there were many boosters that will fit the car available in the salvage yard. caution!!!!!!!do not use one that has been sitting with m/cy removed rain will have gotten in there.

david s poole · #17 10-05-2006, 05:29 PM

ok enough tests.find a booster and put it in the car.the good news is------there were many boosters that will fit the car available in the salvage yard. caution!!!!!!!do not use one that has been sitting with m/cy removed rain will have gotten in there.

rs899 · #18 10-06-2006, 06:29 AM

I agree with David above. I would put money on a brake booster. Any W126 SHOULD fit yours - but pull yours out first if you can and bring it with you to the boneyard or parts store . The fitment issue may be the hole in the fork where the pedal mounts. And , yes, it will come out- the car was not built around it, although it may seem that way.

Rick

BillA · #19 10-06-2006, 08:43 AM

interesting thread
same symptoms, never even checked the actual vacuum
just bought a $69 rebuilt vacuum pump and brakes as good as new
dumb luck I guess (a 300SD is a bugger to stop w/o the booster working)

cousindave76 · #20 10-06-2006, 12:38 PM

So yes I agree... I think I do need a booster. I will hunt one down and post my results when the job is done. Hopfully I'll get this done sometime next week. In the meantime, driving will continue to be an adventure.

I'm tall and lanky rs899. I should be able to contort myself under the dash removing the booster bolts without to much trouble.

byrdandcompany · #21 10-06-2006, 08:18 PM

I am having the exact same problems with my 240d. i am taking it to my "specialist" on Tuesday and getting him to confirm my problem. I stopped by O'reilly's auto parts and they had a new booster for my car for $99.. The cheapest used one I could find was $150. Check them out, they might have what you want.

whunter · #22 10-06-2006, 09:09 PM

Quote:

Originally Posted by cousindave76

Wow... I'm stumped. whunter, you've been doing this a while so I'll trust what you say. Question, when I unbolted the master from the booster and pulled it loose, it sucked in air as though it was holding vacuum just fine. The engine had run about 15/20 minutes prior to me removing it. Does that mean the seal between them is, or was, OK? It's a cheep o-ring so I will replace it as soon as I can find one.
So I'm to the frustrated point now. I have a vacuum pump that put’s out 23" hg, a master cylinder that does not leak a drop of fluid. I could not see any signs of fluid or oil buildup inside the booster when I removed the master. I have freshly bled all brake lines starting on the longest to shortest line. All pistons on all calipers are moving when brake pedal is pressed. New pads new rotors all the way around. And yet I have a brake pedal harder than hell to push.
Benzforlife, looks like I'll have more trouble with this than I hoped.
Could this be as simple as the check valve on the main brake line? How do I know if that is still working as it should? And if it's not, would it create a hard pedal problem?

I'm going to go look in a mirror now and tell myself that someday it will be okay... someday.

I take your word that the caliper pistons are Not sticking.
NOTE: IMO, the four brake flex hoses should be replaced soonest possible.

The vacuum sucking when your master is separated from the booster tells me that:
#1. The booster vacuum valve is GOOD.
#2. The Master to booster seal was GOOD.
#3. The rear booster external seal is GOOD.

Conclusion:
The booster has an internal failure, cause unknown, and not worth further diagnosis.

Best advice:
Replace the booster ASAP...

Any time you begin to think you are having a rough time, read the following thread:

wont start for anything!
wont start for anything! 1985 300SD

It has a happy ending.

cousindave76 · #23 09-24-2007, 07:11 PM

I've been trying to give closure to the threads I have started. This one gave me a big headache but, in the end I came out nicely. My brake booster WAS bad... big surprise. I had it rebuilt at a local foreign parts house here in Dallas for 150 bones. I was nervous that it would not work for a price like that but, for many months now it has been on the car working perfect. Thanks to all who posted to this thread. I would have taken twice as long making sure I was fixing the right thing before I actually did it.

CHEERS!