how to diagnose a no brake problem

If you could not see any brake fluid in the reservoir, then it went somewhere and there is a leak. Perhaps check to see if it leaked out of the master cylinder into the brake booster?

Check both front calibers take each one off and use led light or direct dun light possible fracture I would have never thought air cld get in without fluid leaking I couldn't find no one to help me on my 2000 grand am the brake pad had fell off the foundation so I got tired of putting brakes on got the cheap set after a week the pad fell of it's foundation as I remind u I didn't have a pedal but had to go to work but as long as I pumped them 5r6 times I had a petal let off and gone took off the caliber and held it up to the sun seen a crack a fracture I thought was the seem from the factory turned out it was cracked letting air in but absolutely wasn't leaking no fluid replaced caliber and bleed the lines petal is better that the day I got the car and a quick reminder when changing front pads bleeding the brakes is not required for your not letting no air in ur system