I'm confused. What work was done that started the problem? Did you begin to feel mushiness/travel and decided to change the fluid, or some repair that then resulted in mushiness/travel? Or a preference for the firm feel in the 300SD? You say it's been mushy since '07?

Can you clarify your system - the car is a 190E 2.6 or 3.0? The MC is from a 500E, the front calipers are from an E420 and the rears are from an SL600? And you've got stainless lines on it? And you've pumped 'quarts' of fresh fluid through it?

I had SS lines on an Audi I owned and started getting a mushy pedal and more travel when the lines were about to fail. SS are great when new and fresh - excellent feel, firm pedal - but they wear out just like rubber lines.

But - if I understand your system correctly and the symptoms have been around since '07 - I'd guess that the mixing of components has a lot to do with the lack of feel and pedal travel. You may simply need 1/2 travel to pump enough fluid through the calipers to engage the brakes.