brakes on 2006 Chevrolet Silverado 3500

brake pedal low and soft front rotors and pads replaced only rear pads

It's hard to understand what you're trying to tell me from the way you wrote the question, but I'd guess you may have air in the lines or that the brakes are not broken in yet and therefore feel soft. If you replaced pads and didn't machine or replace the rotors, you'll often get less-than-adequate performance from the brakes.
If you can't get it right, take it to a GM specialist and get it fixed, it's a safety issue.
If you want to find a specialist, here's a directory link for you:
brake pedal was low and soft. Brakes were not working good so we replaced the front rotors and pads and just replaced rear pads but not rotors. The pedal is still low and soft and does not stop well. Just wondering if this is typical of Chevrolets. Our Dodge and other Chevy does not have this problem.