Is there any reason for air to be sucked back into the brake cylinder? I was attempting to flood the system with new brake fluid. Repeated, I mean repeated, attempts to bleed the brake does nothing. The resevoir level doesn't go down as I would expect during the bleeding process. It is an ABS but could the master cylinder be faulty?

There are about 5 or 6 different ways to bleed the brake system, all of them require the reservoir to be kept "full". The best way to flush the system is with a pressure bleeder. Gravity bleeding takes forever! Vacuum bleeders sometimes work. Using a second can with fluid works O.K. Having one person pumping while another operates the bleeder valve takes a while. Whatever method you use BE Careful! brake fluid can be toxic!

On occasion, I have seen the rear of the reservoir become empty without the person knowing.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Well i am using the time tested one pumper and one bleder method. The resevoir never empties and the fluid doesn't squirt out at the bleeder. I think it actually sucked in air since the brake pedal goes to the floor now.

Is something else wrong?

The one pumper & one bleeder method is to have one person pump the brake padle few times and hold the brake padle tight then other person bleed the brake fluid at bleeder valve. make sure you keep brake fluid away from paint.(it will damage car paint) Do it untill brake padle go back to normal high & tight (not soft)
If you still not geting it right,Take vehicle
to shop Only the vehicle is safe to operate,it can be very denger operate a vehicle without brake.(Tow in shop is the best way) have pro to do it with a brake pressure bleeder(much faster) Make sure use DOT 4 brake fluid recommend by MB fectory good lock

Went all the way back to the master cyliner. One outlet won't purge the air the other, the backup, does. Sounds like I found the problem.

Does the plastic resevoir come of the plunger assembly? I pulled a little but didn't want to break anything.

Pull hard and rotate it toward you and the reservoir will come off. The two segments are for front and rear, not main and reserve.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

That makes sense. I presume the front segment is for the rear brakes.

How is the rear part of the resevoir filled? There are two rubber covers there but they seem to be electrical?

By filling the reservoir all the way into the neck area as suggested in my first reply.

I unscrew the cap at the narrow end of the resevoir and fill it. There appears to be a cavity at the rear wider part of the resevoir that is empty. It has a 'min' mark on the passenger side. I would say the two rubber caps on the top in the rear are level sensors as there is an electrical plug there.

So am I stuck on stupid or is there a way to fill this place?

Remove the plastic screen and fill it to the rim. It will bubble and the fluid will seep into the rear via a small hole connecting the two passages.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

I beleive this was my problem from the beginning. The hole is apparently plugged. It has been two days and the chamber has not begun to fill.

Where might it be? I'll clear it out.

Try a new reservoir, Very easy to install & not very $$$$$.

If you are not sure what you are doing it is possible to depress the brake pedal too far. This causes internal damage to the Master cylinder. The result is one half of the master sylinder works, the other half tries to pressurize the resevior on the up stroke. This may be what is causing the bubbles. Only solution is to replace/rebuild the master cylinder. Happened to me as I was using a new helper to bleed the brakes. good luck