Pressure bleeder - how to use on W124?

[taaboo]

Every year I forget about this - when I connect my brake bleeder to our 89 300TE, it won't hold pressure - it vents through the "warning assembly" on top of the reservoir (where the two rubber caps are). Am I doing something wrong? I've used the bleeder (popular one, well made) on several cars without problem.

So I yell for nine year old, to help bleed with the pump/hold/release method, and sure enough, the master cylinder goes. 200K miles and 15 years is pretty good I guess.

Thanks in advance for any tips.

Brian