1987 300SDL, New brake lines,pads (all 4) frt rotors around a 1 1/2 year ago.
165,000mi on vehicle
In the last few weeks I've had to add brake fluid to my reservoir ( 3 different occasions) low fluid indicator warning light has come on.
Today I checked each brake caliper and line to see if they were wet.
They were not.

I also inspected the reservoir area for leakage but could not see or feel any wet spots.

Any ideas of were the brake fluid is going/leaking?

Look inside the car near the firewall and make sure there is no fluid seeping back on the brake rode. I had a Chevy truck leak lots of brake fluid inside just at that precise area.

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Remove the fasteners holding the master to booster, gently seperate enough to check for leak.

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Look underneath car and trace all brake lines to master cylinder. They tend to rust badly above the rear trailing arms.

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1969 280SE
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I did look the lines over while it was on the lift,appeared to be normal/dry.

If I remove the two nuts holding the master cylinder to the booster (and separate) well I have to bleed the brakes?

Is there seals/gasket between the two parts?

As long as you dont disconnect the brake lines that connect to the master cylinder you wont have to bleed... does your brake booster work all right? If there's a leak that bad it would seem that if the MC and Booster seal was bad then your booster would be absolutely drowned with brake fluid.

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1985 CA 300D Turbo , 213K mi

How do I know if the booster is working?
The brake pedal feels normal. No sudden change.

like he says..I had that happen to my W116 after I bought it...slowly disappearing brake fluid..everything remained dry......when my mastercylinder started the sinking pedal thing I found out where it was going...it was leaking into the booster....it was half full (right up to the center hole) of brake fluid...

when you seperate those two you need a new O-ring to seal it or you will have a massive vacuum leak.

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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I recently chased the exact same problem with the SDL. Couldn't find the leak and figured it was leaking into the booster.

But, in reality, the RR caliper had a leak at the seal. The fluid runs down the caliper and down the inside of the wheel and you might not see it. I didn't see it until the leak worsened a bit.

So last nite @ 10:30pm with a outside temp of -2F, I was in my non-heated garage separating the above mentioned items.

I could only pull the master cylinder away from the booster about a 1/2"
for fear of kinking a brake line.
No fluid any where, and I didn't see any gasket either.

I remounted, started the car and pumped the brakes, this time I did notice that there is a slight fade fell in the pedal.
This a good sign of a bad master cylinder?

look closer you will see a squished O ring thats mighty flat.....there is something that seals in the vacuum.

if you have a sinking pedal under steady light to medium pressure get a new master cylinder ASAP before it fails all together. Thats the first sign you usually get when they are failing.....many people never notice it untill it dies altogether.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Ordered a master brake cylinder from FastLane.

When I install it, what is the proper order of bleeding the cylinder.
Is the brass plug on the side of the cylinder used to bleed it?
I do have a pump up power bleeder, but I've never change a cylinder
before only used it to bleed the lines.

Also the SRS warning light come on yesterday while driving (stays on). I did a search and from what I found it has nothing to do with the braking system, correct?

Bleed the master cylinder in a vice, then install and do a normal four wheel bleed.

I would like to thank everyone who posted to this topic.
Thanks for the help.

It was indeed the master cylinder that was leaking into the booster
its was almost ready to start seeping out of the booster.
I used the topsider to suck the brake fluid out of the booster.

As far as the SRS warning light, I found the wire connector behind the air bag
had disconnected.

Again, thanks everyone for your help.