I remember trying to use a vacuum bleeder on a 79 dodge 3 ton with a grain box on it. i opened the vavles as far as I dared to, and still nothing no matter how hard I pumped... so, with one valve open at a time, I got into the truck, and pumped the brake pedal a few times..... Out came stuff that looked like mud with a very thick grease/semi hard wax/grinding paste like texture.... after fluid was coming, I bled it with no problems with the vacuum... funny the brakes didn't seem to stop much better after the change of fluid... the improvement was marginal at best...
I remember practically standing on the brake pedal, and pushing against the roof to get the first pump on each brake to go, after that, it got progressively easier... I had to take the bleed screws out to get that to happen though...
that might be your problem... stuff is clogging the system somewhere along the line and needs pumping out with more force than the pressure bleeder can provide...

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'95 S320 LWB
'87 300SDL
'04 E500 wagon 4matic