The w123 should stop well. I found that my w126 however stopped much easier than my w123 did with less brake pressure.

I would check that there is no brake fluid leaking into the booster. Also, boosters do occasionally go bad and need replacing. Do you feel any power assist at all?

Another option is replacing the booster with a unit from. W116/126 these are larger and provide more assist. There is a threat around sonwhere where sonobe did this. I can't remember if he used the 116 or 126 booster..

__________________
2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold