Hello, Jack from Vegas here. Just purchased an amazingly clean '81 300SD with 190,000 miles on it. Nearly every service record was available, drove it the 300+ miles from Phoenix to Vegas with minimal drama.

There is a few things I want to pick this site's brains on, but right now the most glaring problem occured yesterday.

Coming to a stop the brake pedal went to the floor!!!

Have some mechanical apptitude but have forgotten how to diagnose a power booster or a master cylinder.........

Because it acts like it's not holding line pressure, I'm leaning toward the MC...or should I say I'm hoping it's the MC.

I seem to remember a booster going out on an old truck years back and the pedal was rock hard...

Thanks in advance for all suggestions.

Welcome to the forum!

The brake pedal is assisted by vacuum, but if this went out the pedal would be harder to push, as you suspected. I've never had it go out, but I've read that when it does you have to push harder to brake.

I think you're on the right track.

__________________
Ralph

1985 300D Turbo, CA model
248,650 miles and counting...

MC, calipers and lines.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

Calipers and lines I ruled out because the brake fluid level is topped off.....Yes???

Thanks for the reply.

Thanks for the welcome and thanks for the
reply.

Welcome aboard. There is a ton of great information here and even more very knowledgable people willing to share it with you. I hope you get many years and many miles out of you "new" MB. Great choice!

__________________
2005 Accord Hybrid (Wifes)
1995 Subaru Impreza L AWD (Snow car)
1984 GMC Sierra 1500 (Mine)
1983 300CD Best $ I ever spent. (Mine)
1984 190D (sold and glad I did)
1983 300D (sold and wished I hadn't)

When you press the pedal really fast and hard does it stay hard? But when your gentle just go to the floor?

Good question...........after the "intial" failure, it did re-build pedal pressure.....I believe one time.

Since the first time, however, it now just goes to the floor.

Any fluid loss at all? I had a similar problem in my wagon, it went away after some calipers were replaced.

__________________
http://superturbodiesel.com/images/sig.04.10.jpg
1995 E420 Schwarz
1995 E300 Weiss
#1987 300D Sturmmachine
#1991 300D Nearly Perfect
#1994 E320 Cabriolet
#1995 E320 Touring
#1985 300D Sedan
OBK #42

I would first bleed the brakes,you may learn which brakes are getting pressure or not getting pressure,but I am leaning towards master cylinder,let us know what you find,good luck ,AND WELCOME !

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85 300sd SOLD ,85 745i,95 740i,1972 Suburban,1938 International

Vegas...Welcome

The only times I have had a brake pedal go to the floor and stay there with no pump up response, is when it involved a failed master cylinder. With most modern brake systems there is a dual hydraulic brake circut so if one part of one circut fails (ie. air in line because of a leaking line or brake cylinder) at least the other two brakes should grab...but it might require some pumping. If the MC is not holding pressure then neither circut will work.
Cheers,

__________________
FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K

Thanks all.

I will start with the MC.

Next question..........I'll check the site.........any suggestions where to purchase an MC??

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41VI1BEMMA&year=1981&make=MB&model=300-SD-002&category=All&part=Brake+Master+Cylinder&appChassis=_any

Cool, thanks............now always remember there are no stupid questions, just stupid people.

My VIN is WDBCB20A6BBOO6676


So I ASS/U/ME that my chassis number is <050789???