Whiskeydan,
When your front calipers get stuck, & you say you have to open the lines at the master cylinder, instead of doing that, loosen the two nuts that hold the master cylinder to the booster & move the mc forward away from the booster a little & see if that relieves the pressure. Don't know if Benz has any adjustment, but I had an old Lincoln that drove me nuts till I found that somebody got a little over anxious about pedal free travel & adjusted the nut on the booster pin too far. Or you may have some corrosion in there causing the same result. Paul central FL
'84 300CD