I would nominate the MC first. Possibly something in the assist 2nd.
I suspect you will find that only the rear channel is getting pressure.
Test it. Jack up the front, have someone hold down the pedal while you try to spin the front tires. Although they may grab initially, after a short while you may find that they spin!
I have had 2 MCs fail in this mode (note: neither was a MB).
1986 300E 5-Speed 240k mi.