Things that were not done when the Brakes were done.

The Master Cylinder was not rebuilt
The Calipers were not rebuilt
and the Brake Hoses were not changed.

Just rebuilt a Master Cylinder on my Van, it had a sinking pedal when I kept pressure on the pedal before the rebuild. No exterior leaks anywhere; it had interior leaks due to the seals Rubber seals being shot.

When I had the same issue with my Mercedes I replace the Master Cylinder with a brand new one (Fenco).
While that was a big improvement the Mercedes still seems to stop slower than my other Vehicles as the other Member said about the Mustang it is the same when I drive my Van.

The Non-adjustable Push Rod that goes into the Master Cylinder on the Mercedes seem to cause a noticable Pedal Travel before the it starts to brake.

Having been used to the Rack and Pinion Powere Steering on my Volvo I was suprised at how much extra effort it takes to steer the Mercedes (this could partly be due to the amount of Caster that Mercedes uses) and the less modren Power Steering design.

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84 300D, 82 Volvo 244Gl Diesel