Brake Booster/Vacuum pump Q's - PeachParts Mercedes-Benz Forum

Rmcfarland · #1 07-18-2007, 01:40 PM

Ok, after reading the threads on both of these and testing my system, I am at a bit of a loss. What happened was the power assist on my brakes started going. I would get one pedal push with power assist and then nothing. This very quickly (same drive) went to no power brake assist at all. So I dove into the vacuum system to diagnose.

Here are my symptoms:

Brake booster will not hold vacuum hardly at all. Will creep to ~2mm/hg on the mityvac when running and hooked to vacuum pump.

Vacuum pump on its own will not generate above about ~7mm/hg, with everything disconnected. Seems to drop as I increase idle speed (?)

Vacuum pump will hold 25mm/hg when main vac hose is unhooked and mityvac is connected at that inlet. It may drop a little, I didn't leave the mityvac on very long.

Black motor oil in the main plastic vac line, and around mouth of vac pump and the brake booster.

When the engine is running and the brake booster is hooked to vac pump, with mityvac at the tee in the main line, reads about the same ~2mm/hg that I get with just the brake booster. Drops as soon as I press the brake pedal but comes back up, even when the pedal remains depressed. Keep in mind I can't press the pedal in very far at all.

Engine shutoff holds vacuum just fine.

When engine shutoff is the only thing connected to the tee in the main line, the engine *almost* will die. (when key is turned to shutoff) Shutoff worked fine before my brakes started going.

This sounds to me like BOTH my vac pump and brake booster are faulty! Please, someone tell me it's not true! Let me know if there are other things to look for.

vstech · #2 07-18-2007, 01:47 PM

what is 2mm/hg in inches of water? most guages are graduated in inches of water column most pumps put out around25 or so iWC... I figure around 4iwc for 2mm/hg... sounds bad to me. and the oil in the line confirms it. order a rebuilt vac pump or a pump repair kit.

#3 07-18-2007, 01:57 PM

Why do you think there is a problem with the booster? It does look like your vacuum pump has failed.

Rmcfarland · #4 07-18-2007, 07:21 PM

vstech- the reason that I believe the booster is bad is that it won't hold a vacuum. Unless the booster is set to leak vacuum, but I don't believe this is the case.

Also, all measurements of vacuum were in inches of mercury. Sorry for the confusion. Looked at the gauge again.

Anybody know what might be up with the brake booster? Do they usually fail suddenly or slowly?

I think I will try to repair the vac pump. New ones are very expensive it seems.

#5 07-18-2007, 08:31 PM

I'm not sure what, if anything, is going on with the booster. If it was me, I would rebuild the vacuum pump first and see what happens.

toomany MBZ · #6 07-19-2007, 12:59 AM

Quote:

Originally Posted by Rmcfarland

vstech- the reason that I believe the booster is bad is that it won't hold a vacuum. How did you test? Unless the booster is set to leak vacuum, but I don't believe this is the case.

Anybody know what might be up with the brake booster? Do they usually fail suddenly or slowly? Don't know.

I think I will try to repair the vac pump. New ones are very expensive it seems.

With oil in the line, the vac pump would be suspect.

Rmcfarland · #7 07-19-2007, 01:55 PM

Alright, so I just removed the vac pump diaphram. There is a small tear in it about 1/4 inch long. The rubber has some cracks in it on the engine side as well that don't go all the way through. So, at least there is one problem. I will order a new diaphram today. However, I need the car to get around some before then. Soooo.... looking at this diaphram, I am wondering if I could clean it up and patch it with something until the new one arrives. hmmmm.....will report back when I've got something.

vstech · #8 07-19-2007, 02:10 PM

I can't imagine what you could patch it with that would hold for any amount of time and not damage things further.
if oil has gotten into the booster, orings or seals may have failed. you are going to have a heck of a time testing a brake booster with a hand mityvac... they are REAL big diaphrams, it will take a LOT of pumps to get anywhere.

rrgrassi · #9 07-19-2007, 02:36 PM

Also replace the o ring between the booster and M/C

dieseldan44 · #10 07-19-2007, 03:00 PM

I have tested a failed booster with a mityvac. You get a workout, but if its leaking at all, then it needs to be replaced. With my new one I can pump it up and it will hold vacuum just fine.

I fabricated a connector to mate to my booster out of an old master vacuum brake line hose, are you measuring it a way that you are sure the mity vac is sealing well to the booster?

If you do indeed verify your booster is bad, first, replace the o ring between the MC and the booster. If that doesnt work youll need a new one.

From advice here, I went with an AutoZone rebuild for $115. The quality seems excellent and Im very happy with it thus far. Changing it out on the 123 wasnt that bad either, just be prepared for some twisting to get down there.

Heres a link to my trials and tribulations:
http://www.peachparts.com/shopforum/showthread.php?t=187297&highlight=booster

There are great DIY's in the archives, and the FSM is fairly clear about this task as well. Even the Haynes manual is helpful.

Since youll need to disconnect the MC for this, think about replacing your rubber brake hoses since youll have to bleed anyways.

dd

Rmcfarland · #11 07-19-2007, 03:31 PM

Well, here's what I did. I took the diaphram and cut a piece of heavy plastic sheet (about like a freezer ziploc bag or so) in the same shape and size. I poked holes in it whereever the diaphram had holes around the outside for screws and the one hole in the center. I put the diaphram back into the pump housing with the plastic sheet over it, so the plastic is on the outside. I put it all back together, ran the engine and hooked up the mityvac to the pump. I got almost 20in/hg! From a plastic sheet!

I then hooked up the brake booster and took it for a drive. Seems ok now, brakes feel a little firmer, but that may be because there is more vacuum than before. Anyway, power assist is there, and feels good. If I pump the pedal ~10 times at idle, I do feel the pedal get firm and lose the assist. Is this normal? It comes back soon as rev the car to go anywhere though.

Anyway, I found at least one of the problems. Hopefully I can put in a new diaphram (ordering one today) and all will be well. Certainly cheaper than a brake booster.

NOTE: I am NOT endorsing this repair as a good way to fix your pump. With the plastic on the outside, it shouldn't enter the engine if it breaks, but this is a very lash-up repair. I am just writing what I have done. Hopefully my post tomorrow won't be "Who has a good used OM616?" I think I may take the tube that goes to the top of the air cleaner (the pump exhaust) off, and just let it hang. That way if the plastic lets go it won't enter the intake.

Thanks all for your help. I will write back when my new diaphram is in and repaired properly.

dieseldan44 · #12 07-19-2007, 04:24 PM

My booster still functioned OK even though it was leaking. The vacuum pump was able to keep up with its thirst for vacuum.

But, if only on principle, Id feel better having known 100% functional brake system components.

Oil has gotten into the booster from the vac pump and will further deteriorate the internal seals as time goes on.

dd

winmutt · #13 07-19-2007, 04:53 PM

Did you check for fluid in the booster? I have same problem with hardness at idle. Vac pump is still leaking.... Get a rebuild kite. I have yet to install mine.

Rmcfarland · #14 07-19-2007, 05:08 PM

dieseldan - I have not checked the booster yet for fluid. Can I do this if I remove the master cylinder from the booster? I don't want to have to bleed the brakes if I don't have to. However, there is/has been a small dribble of fluid down the booster from where it meets the master cylinder for quite some time, enough to have eaten away at the paint on the booster. But, I have never had to add brake fluid, and it's right at the full line. Brakes are good on the car. I think I probably need to replace that O-ring.

dieseldan44 · #15 07-19-2007, 05:15 PM

Either way, you need to remove the MC to get the new o ring in there. I guess you could bend the crap out of the brake lines, but that makes me uneasy. Someone will chime in and say theyve done it...

If the booster leaks, Id replace it. Thats my opinion based on my limited experience of replacing my booster once. For me, it was worth $115 + rebleed to know my brake system is tight.

First, remove the MC and put in the new oring. Bolt the MC back on, and test for vacuum. If no improvement then its off with the booster.

Youll need a 1/4" ratcher, a 13mm socket, a universal joint and a 6" extension to get at the bolts under the brake pedal. Took me 20 minutes to extract it after I wedged my head behind the brake pedal.