Brake master cylinder problem

After replacement of brake lines, I tried to bleed the system but after some pumps on the brake pedal, at rear caliper starts to come out only air regardless how long I pump.
Finally, I found that the problem seems to be in the master cylinder (made by ATE). From the front line (secondary master cylinder) its pump only air, whereas the rear line seems to be in order. There wasn't any resistance on the brake pedal.
I disassembled the master cylinder but inside it's looked like brand-new. I also tried a bench bleeding however, without success.
I suspected that the small valve on the front of the piston (that is supposed to release the fluid to the secondary cylinder?) somehow doesn't work even so that this valve also looks good. I tried to flush it with brake fluid but without any effect.
Meanwhile a friend told me that with his 300E has happen the same. Therefore, it seems not to be an isolate case.
I ordered already a replacement master cylinder but I'm still curious where exactly is the problem and could it be repaired?
Any ideas?
'91 MB 300TD turbo (603.960 engine)
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When you pump, don't you mean you pump the pedal 4 times... and then hold... while someone open up the schrader valve .... (pedal goes down)... then close the valve again... and repeat?
If not, then ya, you are pushing and sucking air at the same time.
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I found where the problem is...
It's not the master cylinder.
The brake fluid tank consists of two separate compartments - one for the front and one for the rear wheels. The two compartments communicate via very small orifice close to the upper wall of the tank. In order to get fluid to the second compartment one must fill the front compartment TO THE TOP and even to tilt and shake the tank, and to repeat this several times until the rear compartment is also full. Then the bleeding of the rear well can continue.
I read about the bleeding procedure in several books but nowhere was even mentioned that one must maintain the tank always full to the top.
Well, now I have my brakes again and a spare master cylinder in very good condition.
Thanks
Dimi
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