When you appy the brakes is the pedal "hard" or "soft" and how much travel do you go through?
Air would tend to let you push the pedal further to compress the air, making the feel more on the soft or spongy side.
It is also possible the new discs came with a preservative that is interfering with your pad/disc ability to convert kinetic energy to heat, and until it wears off you will have a less effective braking system. I think you are supposed to clean them (the discs) off when you install them.
If you get the chance, call the outfit that sold the discs to you and ask them if they ship with a preservative. Since most discs are made of non-corrosion resistant steels, it is not uncommon to coat them with something to prevent them from degrading in storage or shipping. Like wet brakes, until the material is removed, it acts like a lubricant, making your foot push harder to get an acceptable response.
If you cannot "burn" the stuff off by driving around with the brake applied lightly, you will probably have to take the calipers off to wash the stuff off so you do not contaminate the pads.
Based on what you wrote that is about the best I can do, hope it helps, and good luck. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)