MC and or Brake booster help! - PeachParts Mercedes-Benz Forum

jonschwenke · #1 03-07-2011, 09:43 PM

I've done quite a bit of searching, but cannot come up with the info I need. Please help.
I rebuilt the rear calipers on my 84 300tdt wagon. Bled the system and my MC failed ( I think). Replaced it with a rebuilt unit. Bled the system til blue in the face both with a homemade pressure bleeder and with the assistant pedal pump. I did bench bleed the MC, and after installation cracked the lines with pedal assistant to bleed again. also at the calipers in order Pass rear, Driver rear Pass front Driver front - extensively. When the car is off, I get slightly spongy pedal, but it will not hit the floor or sink with light pressure. There are no brake fluid leaks that I can find. When I turn the car on, the pedal drops considerably and nearly hits the floor. I interpret this as a brake booster issue...right? I grab a vac gauge and put it on one of the small lines coming off the vac line to the booster. Start the car....15hg slowly. I step on the brake and vac dissappears and then is erratic. Upon release of the pedal vac climbs very slowly to 15hg again. I disconnect the line from the booster, plug the end... 20hg immediately. I stop the engine and 20hg holds. I interpret those results as Vacuum pump is ok, maybe a little tired, but fine. So, more pointing to the booster right? Or maybe the valve in the booster vac line? The o ring seal between the MC and booster is new and appears to be installed properly and sealed. If there's something I'm missing or some other tests I should perform please help. I'm not sure if the Booster is supposed to behave that way or not. As far as the MC, it is a rebuilt unit and it's possible to get a bad one, but I'd like to fix this brake booster issue ( if it is indeed one) first. One other note, I have been experiencing some issues I believe to be vacuum related such as door locks, and climate control not quite operating properly. If check valves are faulty or not installed correctly and the booster is leaking, could it be the culprit? Thanks in advance.

tangofox007 · #2 03-07-2011, 10:10 PM

Quote:

Originally Posted by jonschwenke

When I turn the car on, the pedal drops considerably and nearly hits the floor. I interpret this as a brake booster issue...right?

No.

Any chance you reinstalled your calipers on the wrong sides? Doing so places the bleeder valves at the bottom instead of the top.

jonschwenke · #3 03-07-2011, 10:21 PM

Caliper bleeders are on top indicating correct orientation.

1stbenznc · #4 03-07-2011, 10:45 PM

Pedal should get hard when car is off OR when it is on and pumped. It's not the booster. Booster just aids power braking, not stiffness of the pedal. Sounds like you bleed it all very well. No bubble or spitting, right?
I think it's a bad Master cylinder, unfortunately.

jonschwenke · #5 03-07-2011, 10:57 PM

Quote:

Originally Posted by 1stbenznc

Pedal should get hard when car is off OR when it is on and pumped. It's not the booster. Booster just aids power braking, not stiffness of the pedal. Sounds like you bleed it all very well. No bubble or spitting, right?
I think it's a bad Master cylinder, unfortunately.

Sounds (and feels) like I may have both MC and Booster issues. I can re-bleed everything, but can any one tell me...do my vac tests indicate a bad booster? Also, does anyone know how to test the booster please.

leathermang · #6 03-07-2011, 11:02 PM

You have to fix the MC before you can really tell if the booster has problems....
and typically ... if you come to a stop and it slowly sinks to the floor.. or near it... then you have a leak somewhere in the booster...

jonschwenke · #7 03-07-2011, 11:03 PM

One other thing I forgot to mention. There was fluid in the booster when I took off the first MC. Looked super old and nasty, but not much of it. I sucked it out with a siphon and carefully ragged up the rest.

1stbenznc · #8 03-07-2011, 11:35 PM

The fluid in the booster was definately from a bad master cylinder leaking into the booster. Depending how much fluid got in there, yes, it may have messed up the booster too. That fluid is very corrosive. Don't know about the vacuum pressure for the booster - sorry

jonschwenke · #9 03-08-2011, 12:42 AM

Quote:

Originally Posted by 1stbenznc

The fluid in the booster was definately from a bad master cylinder leaking into the booster. Depending how much fluid got in there, yes, it may have messed up the booster too. That fluid is very corrosive. Don't know about the vacuum pressure for the booster - sorry

The fluid was so nasty I think it may have been in there since before I bought the car in 2005, or a very slow leak since

! I have never seen a fluid level change in the reservoir.....

jonschwenke · #10 03-08-2011, 01:53 AM

Just found this after a google search....Nice info on the brake booster here!

http://www.autoshop101.com/forms/brake05.pdf . Maybe should be in the DIY section since it gives great info on checking and adjustment. Alright, now I'm hittin' some hay.

Stretch · #11 03-08-2011, 02:53 AM

Quote:

Originally Posted by jonschwenke

Just found this after a google search....Nice info on the brake booster here!

http://www.autoshop101.com/forms/brake05.pdf . Maybe should be in the DIY section since it gives great info on checking and adjustment. Alright, now I'm hittin' some hay.

Thanks for posting that link to the pdf - it is indeed good information. As for being in the DIY section I don't think we can do that for copyright type infringements...

...I guess the next best thing would be to ask whunter to include the link in one of his great lists

toomany MBZ · #12 03-08-2011, 09:20 AM

Agreed, very good info there.

If you can get an adapter hooked up to the booster vac orifice connected to a mity-vac, pump away, this will take some time, but does it hold vacuum? If so, the booster is good, if not, the diaphragm has been compromised.

jonschwenke · #13 03-08-2011, 02:04 PM

Thanks for the tip toomany. Makes sense. However I called around, and found a reman booster from a parts store for about $82. I'm going to install the new one and be done with it then turn my attention to the MC. The old one has been on there for 26 years or so, and even if it is still good, it won't last forever. I'll see if I can test the old one on the bench and if good keep it as a spare. I'll report back with my results.

toomany MBZ · #14 03-08-2011, 02:36 PM

Okay, you had fluid in the booster, you'll need a new master cylinder to go with that reman booster.

jonschwenke · #15 03-08-2011, 09:55 PM

Only had a couple hours today. Removed old brake booster and installed the reman unit. I bench bled another reman MC. Removed suspect first reman MC and installed the newer one. As my wife was calling for me to watch the kid while she got ready for work, I stepped on the pedal to find it already much better. A little spongy, but I didn't re-bleed the system after the newer MC install. I'll bleed tomorrow afternoon, and I'll remove the rear calipers (which is why I started this mess), and rotate them while bleeding to make sure no air is trapped there. I'll use care and blocks of wood to ensure the pistons don't come out of the bore. Seems the first reman was a dud.

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