Within the last month I have been leaking brake fluid in my 1983 300D. Now I am topping it off every other week. I pulled each tire and there was no wet spots anywhere and the pressure release valves are not wet. The reservoir does not appear to be leaking underneath. Yet I know it's leaking. Where can it be leaking from?

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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
99 ML 320 Black (lease), 1998 C230 White (lease)
00 Honda S2000 Red (lease)
86 Mercedes 300E (sold)
84 Porsche 911 Red (sold), 1965 Porsche 911 White (sold)

Is it leaking equally across both sides of the reservoir? If not, that helps narrow it down - if it drops only in the front half of the reservoir the leak is in the rear braking system, and vice-versa.

If it's leaking down equally, and you're not seeing any fluid anywhere, then the odds are pretty strong that the seal on the end of the master cylinder that is inside the brake booster is leaking. To check this you'll loosen the two nuts that hold the master cylinder to the brake booster and pull it forward to see if you can spot any wetness inside.

Note that there is a rubber seal ring between the master cylinder and brake booster that is there to keep air out/vacuum in. Make sure this gets re-seated properly or you'll have a vacuum leak and hard brakes.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

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There is a at th rear of the Master Cylinder that can leak and Brake Fluid can go inside of the Power Brake Booster.

There is also a outer O-ring on the Master Cylider Flange that seals the Brake Booster off from the atmosphere so when the seal in the back of the Master Cylinder leaks fluid you do not see the leak because it cannot leak to the outside.

See post #6 for a diagram of the Master Cylinder.
83 300td no brakes hissing from vacuum pump

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84 300D, 82 Volvo 244Gl Diesel

How is your brake pedal? Is it soft? If so your master cylinder either needs to be rebuilt or replaced. Or if your pedal feels firm, then the seal is leaking.

The brake fluid is most likely filling up your brake booster. It would be best replace the seal between the master cylinder and the brake booster. As well as remove as much of the brake fluid as possible and clean it out. Carry out this procedure as soon as you have a new seal, or master cylinder, as failure to do so can and will lead to failure of your brake booster.

You will need to bleed the brakes to do this procedure, dot 3&4 brake fluid. Build your self a pressure bleeder, and this job will be relatively simple.

I followed this guild to build my pressure bleeder: The DIY $20 brake bleeder

FYI: A New master cylinder (only buy ATE (oem supplier) or OEM). Will come with a new sealing ring.

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Nosce Te Ipsum "Know thyself"

If you happen to have a W123 that has ABS brake system I have no info on it.

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84 300D, 82 Volvo 244Gl Diesel

Thanks guys. I just topped it off so I will have to wait to see if it is lower on the front or back of the reservoir. I didn't see a partition in the reservoir? The pedal is firm but the travel is longer over the past month. So do I just get a new master cylinder seal and install and bleed the system? No ABS in the 83, I think only the 85 has ABS brakes.

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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
99 ML 320 Black (lease), 1998 C230 White (lease)
00 Honda S2000 Red (lease)
86 Mercedes 300E (sold)
84 Porsche 911 Red (sold), 1965 Porsche 911 White (sold)

Both my 300D's had brake fluid inside the booster. It was a bit hard to get it out since it doesn't all run out under gravity. I had to stuff in rags to absorb it. Each car was not being used for months during this, so I had leisure to do it right. Both cars got silicone fluid (DOT 5) w/ their new MC's, so little concern w/ rust, which is the main thing that causes the seals to fail.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

The partition in the reservoir is there, but not readily visible. It works in such a way that as you fill the reservoir only one half fills until the fluid level reaches the top of this partition, or baffle, and then the second side starts to fill. Then, at some point, both sides fill equally as it's above the baffle.

It may be hard to tell if just one side is leaking if you're topping off regularly as under the same principle as above fluid level drops equally across both sides until you've gotten the level below the baffle at which point the fluid would then only lower on the side the leak is on (assuming it's a leak at a line or caliper). Just to note, in case you weren't aware, this design it to ensure a leak from something like a ruptured brake hose only takes out 1/2 of your brake system. It's a safety measure.

That said, generally speaking when you're losing fluid but you can't see where it is going it is most likely leaking out of the back of the MC.

If you find it is leaking, I'd say your best bet is to pick up a new or reman master cylinder. Inexpensive, decent quality remans with lifetime warranties can be found at Napa. New ATE master cylinders are a bit pricey but another option.

And just to clarify something else, as someone who owns an '85, ABS was only an option in that year. It was not standard.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!

I am not sure if the seal there is just an O-ring or is actually a lip type seal. A rebuild kit for the Master Cylinder costs more then an exchange M Cylinder dose.

Also if you take a close look at the drawing of the Master Cylinder in that other thread I posted you will see that even if you had a Frkee Master Cylinder rebuild kit the Piston that is in the front has a Pin going throught it that has to be pulled out in order to get the front Piston out.

Note: If you replace the Master Cylinder be picky about how it works after it is installed. I installed a Cardone Rebult and it had a tiny bit of sinking pedal. 1.5 Years later the Master Cylinder failed completely.
I ended up buying a brand new Mastery Cylinder made by FENCO (Taiwan) because at the time the price was good and have had no issues since.

When I found the paper work for the Cardone Master Cylinder I when back and got another under the warranty. But, it just sits in the Box.

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84 300D, 82 Volvo 244Gl Diesel

my lines at the rear where they go over the sub-frame developed rust and were weeping. Really hard to find when there is a lot of dirt and grunge in the space between the body and sub-frame. It became VERY apparent when the fuel lines started leaking in the same spot for the same reason... (Former New England car) - MC most likely, but there is another place or 2 to poke at... Good luck!

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1985 300D
The rest:
1957 MGA (comatose)
1965 Falcon (sleeping)
1966 E-100 (rust test in progress)
1976 Ford 3400 D Tractor (workhorse)
1978 Mercury Zephyer (5L playtoy)
1995 Isuzu NPR D (fetcher)
1998 Subaru Legacy (Spare)
2000 Toyota Sienna (School bus)
2008 Toyota Prius (Commuter)

Yep, leaking into the booster, new MC time.

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83 SD

84 CD

If it is leaking into the brake booster, you'll have to remove the booster to remove all the brake fluid. You don't want to let the brake fluid sit in there. Brake fluid readily absorbs moisture, and with old brake fluid in the booster, can cause the booster to rust from the inside.

Might as well do it properly - then bleed the MC and calipers for good measure, and peace of mind.

Here are pics of the brake booster innards, found in this thread.


Left: front cover
Middle: diaphragm (?)
Right: rear cover



Diaphragm:


Close up of diaphragm and rear cover:

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http://farm8.staticflickr.com/7271/7...144c3fc1dc.jpg

Thanks, MBeige, does that brake booster come apart readily?

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85 300CD Signal Red/Tan sold
83 300D Manganese Brown 109K
97 E420 Midnight Blue 197K sold
98 BMW 328i Vert White 100K, sold
95 BMW 525i White 125K, sold
93 BMW 525iT Red 193K, sold
95 E320 Green Wagon 125K, sold
94 E320 White 127K, sold
85 300SD 156K Grey (Annie), sold
84 300D Lapis Blue 170K (Judy), sold
99 ML 320 Black (lease), 1998 C230 White (lease)
00 Honda S2000 Red (lease)
86 Mercedes 300E (sold)
84 Porsche 911 Red (sold), 1965 Porsche 911 White (sold)

I don't think so. I left it to my mechanic to tear it apart, you can see the areas where he had to separate the shells by undoing what seem to be crimps.

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http://farm8.staticflickr.com/7271/7...144c3fc1dc.jpg

whats the broken part?

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!