Air in the system or a leak.
Bleed the front brakes again -- you can use gravity by attaching a plastic hose to the bleeder and running it into a container. Just open the bleeder a quarter turn or so and keep the reservior full.
Most likely you either allowed the reservior to get too low or didn't bleed long enough to get all the air out.
Bad vac will give you a very hard pedal, but the brakes will work fine if you apply enough pressure to the pedal. Soft pedal with excess travel is air or a leak.
The last possibility is that when you bled the brakes and the pedal went all the way down the master cylinder seals failed -- the intake bores on the master cylinder aren't chamfered on MB master cylinders, so a bit of rubber from the piston seal can get nipped off. Ditto for dropping the piston to the bottom of the bore -- any roughness there form ancient brake fluid will chew up the piston seal, and the pedal sinks.
Quick test is to rapidly pump the pedal -- if it suddenly gets firm and the brakes work, but goes soft again after release, the master cylinder need a rebuild kit or replacement -- some have a kit readily available and some don't.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!