No need to bleed the abs system separately, Mercedes recommend you do not go above 29psi when using a pressure bleeder so you can try a higher pressure than you have been using. Always start by bleeding the longest line first (passenger side rear) and finally the shortest line from the master cylinder.
This may sound elementary;but I have seen experienced people do it; do not shake the bottle of brake fluid to mix it before use as all you will do is introduce air into the system.
If your brakes still feel spongy i would think you may have a worn master cylinder or soft flexible brake pipes.
Hope this helps,
Geo.