Well, I did it to myself, with the kind help of the SDL.

I'm fully aware of the two chambers in the M/C. Naturally, I believe that the front chamber controls the front brakes and the rear chamber (the smaller one) controls the rear brakes.

But, what I was not aware, is that the rear chamber does not extend all the way across the reservoir. The dam in the middle extends from the right side of the reservoir toward the left side and then it makes an abrubt 90 degree turn to the back of the reservoir.

So, when you look at the reservoir from the driver's side, you see only the front chamber.

To see the rear chamber, you must look at the reservoir by leaning into the engine compartment and looking at the reservoir from the passenger side. Then the separation of chambers is visible and the level of fluid in each chamber is visible.

Why all this BS you say?

Because I must have had a small leak in the rear brakes, after doing the brake job four months ago. I noticed the reservoir level drop slightly, but, never took much stock in it. I'll check it out when the weather gets warmer.

This morning: Lost half the system. Reason: no rear brakes

After pondering a new m/c, I determined to find out whether I'm not seeing the fluid levels properly in the reservoir, and, well, you know the rest of the story.

So, my question to those who have stuck themselves in the same situation and resolved it:

Can you bleed the rear brakes at the calipers and get away with it? Or, must the master be bled first and then the calipers?

If the master has to be bled on its own, can it be bled on the vehicle? If so, can just the rear brake system be bled?

TIA.

Sorry about your brake troubles, but thanks for the heads up on properly reading the brake fluid levels.

If you use a pressure bleeder, you could conceivably get by without bleeding the master cyl first. A MityVac bleeder may also work, but the old pump n' blow method would probably take a lot of time, and could also damage the internal seals within the master.

Where exactly is the source of fluid loss? Any obvious external signs, or is the master possibly leaking back into the vacuum booster canister?

I use a homemade pressure bleeder patterned off of this one:
http://www.bmw-m.net/TechProc/bleeder.htm

I've got an "easibleed" that can bleed them with about 25 psi from an old tire.
It works pretty well and keeps the m/c full throughout the process.

My guess is that one of the rear caliper bleeders is probably leaking a touch. But, I haven't had the rear wheels off since the brake job in October.

Thanks for the info Z

Brian, you may recall that a few weeks ago, I posted my frustrations with the rear reservoir being difficult to read. Went much of this winter with just fronts working as I had been under the impression I needed to replace my M/C.

Anyway, I used my Mighty vac to pull fluid from the rear B/C's while adding fluid to the reservoir. I had to rock the plastic tank back and forth so as to facilitate keeping topped off, the rear of the M/C reservoir.

This process worked fine for me! BB

Thanks BB. I do recall those discussions and I clearly remember the two chambers on the SD. But, the SDL is different. The rear chamber can really fool you. You cannot see it from the driver's side of the vehicle.

Is the W124 the same?

I'll give the easibleed a whirl. It maintains a fluid level right up to the cap and will introduce no air unless one runs the bottle out of fluid.

Hopefully I get lucky with it.

Can't recall on my SDL, it's gone. But I thought all brakes on the Mercedes were set up diagonally. Front-left right-rear and the other chamber supplies the front-right rear-left. Could be wrong on this for the SDL.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

I don't think so Dave. This would send the vehicle into a spin if you lost one system. The rear calipers have smaller pistons, IIRC.

Additionally, the rear reservoir is about 1/4 the size of the front. Presumably, the rear brakes use less fluid overall.

Brian you are correct. I reviewed the parts manual for the W123 and it shows a front rear split. However a diagonal split was done my Mercedes.

http://www2.mercedes-benz.co.uk/content/unitedkingdom/mpc/mpc_unitedkingdom_website/en/home_mpc/vans/home/products/new_vans/sprinter/panel_van/prices_and_equipment/standard_equipment.html

It is also discussed in my SAE study guide. Many manufactures set up the brakes with a diaganal split.
Why I remembered, incorrectly, that the SDL had a diagonal set up I don't know.

Live and learn.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car

Wouldn't it be a requirement to have four identical pistons in order to use a diagonal setup?

If you had four identical pistons, wouldn't you have too much stopping power on the rear wheels?

Brian,

This is what proportioning valves are for. Identical pistons on each of four wheels, fed from identical master cylinders. The valves direct most of the stopping force to the front, where it belongs.

__________________
Alan the Radio Guy
--------------------
Amateur Radio Station N7CEU
1985 300CD 199,000 and still rolling

Yep, kind of forgot about them. I remember their requirement when drum brakes are used, but, presumably, they can be used with discs as well.

Do we have them on the W123 and W126 vehicles?

I'll see if I can get a photo of my 124 reservoir and post it. Interestingly too, I'll look at the SL's reservoir too. I just cleaned 25 years of 1/8' baked on sludge off it's opaque white reservoir. It must have the dam too!

Some VWs have diagonal brake split. The steering geometry is set up to counteract the effect of having one front brake working. It's called "Negative steering roll radius."

The idea is that you always have at least one front brake working, which is important in a nose-heavy front wheel drive car.

__________________
1982 300SD

They called it a dual diagonal brake system.