I have a 2003 Chevrolet SSR (Super Sport Roadster) pickup. Sometimes when I get in and push on the brake pedal (key off) the pedal goes to the floor. Take foot off brake and start...Push on pedal and it goes about 1/2 way down. It will stop the truck.

Next day or two go out and do the same thing....step on brake without key and the pedal is nice and firm as it should be. Take foot off pedal and start engine and the pedal feels normal.

This is an intermittent happening. Maybe about once a year. Any suggestions? No leaks in system and the M/C is full.