I had spongy brakes and they replaced the master cylinder. Now I have a soft brake pedal, meaning they worked but the pedal goes down farther than it ever has. What could be causing this, air in the line? another leak? didn't see a leak. This is on a 06 chevy silverado crew cab 1500 2wd
There's air in the lines from the sounds of it, likely from them not correctly bleeding the system. An air bubble in the system will compress before the rest of the fluid in the lines, thus not actuating the brake callipers.
If you don't have the tools to bleed your brakes at home, you should take it back to the workshop and get them to asap.