So my 05 Silverado needed new brake fluid badly(also had about 3 inches of pedal movement(shut off)), so i decided that was today's project. So i bled all the brakes, everything went well. but my problem is that when the truck is running the pedal movement is at about 3.5-4 inches, but when it is off, the pedal is phenomenal, at about 2 inches, i know the brake booster sucks the pedal down more, but does it need to go down that far? any help would be much appreciated, thanks!

 

I'm thinking the master cylinder or the vacuum booster is bad...
the fact that is behaves differently without power leads me to the booster...
but I'm no brake expert and can only go by the description you have given...

You could still be sucking air into the system... where did that fluid go? is one of the cylinders leaking?

 

Thats a closed loop hydraulic system. If the fluid is low you have a leak, either internal to the booster or external. I too suspect the MC as it can leak inside itself across the cups, that would be indicative of the low pedal, but wouln't explain the low level.

 

if its a vacuum booster i'd be unbolting the master cylinder to see if its leaking inside the booster