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Old 10-30-2000, 05:07 PM
Ron D. Harriman
Posts: n/a

Not asking a question here; relaying a
personal experience instead.

Bled the brakes for the first time myself
on the 380SL. I typically do one circuit
at a time and check after each caliper.
(I use a Mityvac vacuum tool.)

Drained master cylinder res, refilled, went
to first wheel (psgr rear) bled it, closed
up, went to check pedal feel -- overtravel.
Spongy as hell. Arrrrgh! How did I get all
that air in there?

I blamed the Mityvac and futzed with it
until it was too late to futz. Today, I
checked the shop forum for leads. Instantly
found a note to the effect that there are
two sections inside of the master cylinder
reservoir, that interconnect at the top with
a tiny pinhole.

Filling section 1 doesn't get fluid into
section 2 unless 1 is all the way to the top,
according to those posts.

Checked this car. Yep. Empty section 2.

Filled to the top. Section 2 filled up
once I did that. Rebled. Firm brakes.

Two observations: (a) boy, you guys rock.
I would have probably had to have the SL
towed to a shop if I hadn't found that note.

(b) What are MBZ engineers smoking in their
off hours? Dumbest M/C reservoir design I
have ever seen. And potentially dangerous.
Safety critical systems should not include
stupid pet tricks. Obviousness, please!

I hope I haven't torn up the M/C seals
with the pedal overtravel. No way to tell
until it fails on me. At least this way
I can hopefully have someone else learn
from my experience.

Ron H

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Old 10-30-2000, 08:16 PM
Registered User
Join Date: May 1999
Location: Charlotte, NC, USA
Posts: 688
Ron H,
There is some sanity remaining in Germany and what they do is a safety factor. There are two reasons for this design.
First, is in case you lose a brake hose or some other important part, you still have 1/2
a system left to slow down and not crash!

Second, is because there are many standard shift cars in the world (not us) and the brake master cylinder reservoir is also used to supply fluid to the clutch master cylinder. They cut costs by using the same design for all cars as there is a port at the back of the reservoir to slip a hose on.

Now, you wouldn't want to lose all your brakes because of a faulty clutch slave cylinder, would you?

Tobias MB
C280Sport, 300CE, 400E, 190/5.0
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Old 10-31-2000, 05:55 AM
Posts: n/a
As Tobias said, their not quite ready for the funny farm in Stuttgart.

In the mid sixties, "dual master cylinders" became a new safety feature in all cars. By having seperate cylinders,(actually two in line cylinders and pistons) and two reservoirs, a leak in one, does not render the other useless.

If you only had one reservoir, a leak anywhere in the system would lose your fluid and eventually ALL your braking.

The way that the Benz reservoir is arranged, it's not as obvious that there are actually two reservoirs. On many American cars it is more obvious.

Good luck,

Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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Old 10-31-2000, 06:44 PM
Registered User
Join Date: Nov 1999
Location: Germany
Posts: 63
good discussion, but I still don't understand the pinhole connection between the two reservoirs....if one system leaked, wouldn't the pinhole allow pressure to bleed out of the good system?
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Old 10-31-2000, 06:52 PM
engatwork's Avatar
Join Date: May 2000
Location: Soperton, Ga. USA
Posts: 11,246
yea it would but only at a VERY slow rate.
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Old 10-31-2000, 09:45 PM
Registered User
Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
The pinhole is near the top. You really have to fill up the front reservoir very full to get the brake fluid to the rear reservoir. I do not see how the pressure of one reservoir can leak thru the pinhole to the other one. There is no pressure inside the reservoirs.

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