If you've verified the pistons are moving in the calipers, then the calipers are not getting pressure.

Note: vacuum bleeding is ineffective on ABS equipped cars.

While you have a "volunteer" holding pressure on the master cylinder, go under the hood and crack the lines at the master cylinder and the
ABS behind the left front headlight.

It's not unheard of for the ABS unit to clog and prevent pressure from reaching a brake caliper. If the unit is clogged, it will be obvious when you crack the lines. If the pressure is weak in both the gozinta and gozouta lines, then the problem is probably the master cylinder.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles