i replaced the brake master cylinder in a friends 1984 300SD today.The car had no brake pedal and fluid was leaking down the power brake booster.After bleeding the system on the bench and at each wheel the car has a very hard pedal yet does not stop.Put the car on a lift found the rear brakes donot work properly.So i took out one of the pads and pushed the pedal to see if the caliper pistons were moving.That one was,so i removed the brake pads(good pads front and back) cleaned and greased all sliding points.Took the car for a ride and it still barley stops.Am i doing something wrong or not doing some thing.You guys are the authority on these fine automobiles so any help would be appreciated.
thank you in advance for any help at all
James

Sounds as if the power booster isn't functioning. Are all your vacuum lines/tubes properly attached to the booster/manifold?
Is the valve rod from the pedal assembly lined up correctly in the control valve?

Did you put the rubber O-ring seal on the master cylinder body before installation? This seals the cylinder to the booster and allows the vacuum to build in the chamber.

Car is fixed
 

Thank you for you're help guys.Left the o-ring off the master as steve said.Replaced it in 10 mins and car feels great.
thanks again
james