brake petal went to the floor on 2003 Chevrolet Silverado 2500 HD

Brake petal is very soft, still have some brakes. replaced master cylinder and problem is still there

Asked by for the 2003 Chevrolet Silverado 2500 HD
You may not have gotten the air out of the brake system, especially if you have ABS brakes. If you do, there's a 'Scan Tool Bleeding Procedure' which requires a scanner to bleed the ABS system. You may need to go to a GM specialist with a scan tool to get the brakes bled.
Here's a directory link for you: