brake pedal

[bobfish45]

Hope someone can help with my irritating problem.When the engine is off,the brake pedal is solid,turn the engine on,and the pedal will sink all the way to the floor when pressed hard.The brakes work fine,and the only work I have done is changing disks,pads and brake fluid.Could I have damaged any seals when pushing the pistons back in?It has always been ok before,but,I don't know.Thanks in anticipation,Rod.