Hey chicos, are the dual diaphragm W126 brake boosters considered more reliable then the monophragm W123 and old dualphragm W116 boosters? Or did I hear wrong? Also what is considered an early W126? The early ones had the same booster as a W116 I heard.

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A junkyard is not for junk

I have an 84 & 85SD. At least 1 is running on the original booster at 250,000 might. I consider that sufficiently reliable that I'll simply replace or build if it fails. There isn't a need to consider "most reliable " given the mileage.

I agree. They're very very durable in the single diaphragm booster. I have no experience with the doubles.

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[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.

My '86 SDL has the original double booster on it and shows no signs of having problems. My '83 500SL has the original single booster on it and again, has no issues. I wouldn't say one is more reliable than the other. Remember that most cars have a single diaphragm design.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

How dangerous is a brake fluid leak from the master though? My single on my w123 had the paint disintegrate beneath the master but it still worked damn fine. Would you need a large volume of concentrated fluid to dissolve the diaphragms? Its one of those things the PO overlooked, and I'm wondering if I just got lucky or if this leak wasnt actually as dangerous as it looks.

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A junkyard is not for junk

You're more at danger from low fluid level than booster failing. Even if the diaphragm goes "pop" you're not going to lose braking ability. Increased effort? Yes. Loss of brakes? No. Single or dual diaphragm boosters will be affected similarly by fluid leaks

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

I think if one diaphragm fails it dose not matter if the if it is the single or double diaphragm the brake booster is not going to work correctly.

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84 300D, 82 Volvo 244Gl Diesel

Ok so for you pick your part'rs, here is an example of a bad boost and a good booster. The booster with the corrosion on the lip where the M/C connects has obviously had a fluid leak and the brake fluid unfortunately ruined the booster. This booster passed all the vaccum tests I could do with a hand pump, but unfortunately, after even one millimeter of brake pedal travel, all vacuum would be lost and the brakes would go rock hard, like pushing a stack of cinderblocks with your heel.


I pulled two other boosters from the pick your part, and both had cleanly painted lower lip/rims, and both work flawlessly. A piece of advice I would give, and am currently giving, is only buy a used booster if that inner rim/lip still has clean paint and no evidence of fluid leaking in.

Here's a good picture of one of the great boosters, this one is currently installed in my W123 and is quite powerful! It is quite nicer than my original attempt with the broken booster that I sanded, painted and buffed. *sigh*


Again, a clean painted lower lip/rim where the M/C goes through indicates that the Master Cylinder (M/C) O-ring seal was still holding vacuum and keeping fluid out. A fresh and nicely painted rim is quality insurance for a good booster.
Definitely do not buy a booster with a corroded lip! It'll just be a big shelf piece!

As for the rust around where the master cylinder bolts to the face of the booster, that is likely caused by water that gets between the M/C and the Booster, but does not make it past the seal. It is perfectly harmless. Or as least as harmless as rust gets anyways.

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A junkyard is not for junk