Right after I bought my W111, I pumped grease until I was blue in the face. I reached the same conclusion as you and stopped. I read more about it and decided to give it another "shot" (sorry for the pun).
It took several more pumps but FINALLY some horrible looking grease came out of the check. I doubt that had been done in ages. I am not entirely sure about the depth of the cavity. But now that mine is filled, I just pump a couple of shots and the grease comes out the check. I do not see any grease escaping, so it must be captured.
The vent is actually a spring loaded check valve that only allows grease to escape when it reaches a certain pressure. Based on the ring of old grease on the driveshaft tunnel right in line with the vent, a little grease probably oozes out when spinning.
So pump it up with confidence!