An easy test for restricted hoses.

Jack wheel
Push pedal down hard once
Turn wheel by hand
If hard to turn, open bleeder
If wheel frees up w/bleeder opened , hose is bad .
If wheel still hard to turn, caliper is frozen , or if rear brakes, could also be bad emergency brake adjustment/cable.

If all wheels do it , possible Master or pivot of the relay lever behind booster
is sticking.. [ this is where the pedal rod comes to push the booster in..common on older Benz and a new pivot pin kit is available - or lube]
As previous post states, hoses are always first suspect on an old Benz..they look great from the outside , but restrict on the inside to the point of holding pressure on the pistons..same as riding the brake pedal...