Malibu Brake Pedal

I just replaced the rotors and pads on my 2002 Malibu. I had moved the pistons back with a C-clamp to make room for the new pads - no fluid spilled from the master cyl., didn't open bleeder valves. When I went to back off the ramps the pedal went to the floor unexpectedly. After pumping the brakes a few times the pedal was firm. The fluid level was OK. I've driven 7 or 800 miles since then and today while I was driving the pedal went almost to the floor then the brakes worked. Then, the pedal was normal, then went to the floor - intermittent. The pedal is firm when it catches, I just don't know how much the pedal is going to move till it does. The master cyl. fluid level is still OK. Does anyone know what this could be? Thanks!

Air may have passed by the seal when you pushed the piston in. If you don't have any leaks then it must be an air bubble. I'd give the system a good bleeding.

Just replaced the front rotors , pads and calipers- bleed the brakes and now the pedal goes almost to the floor. Rebleed all of the brakes about 12 times per wheel and no change. Is there something that needs bleed on the abs system?