Brake Problem

1984 pontiac parisienne station wagon. Sometimes the brake pedal has full pressure, and sometimes i get no pressure. Sometimes when stopping it will have full pressure but while sitting idle with the brake on, the pedal will go to the floor and the car lurches forward. I thought it was a union fitting leaking so i replaced it and bled all 4 brakes but still it does the same thing! Its getting kind of dangerous, any ideas where i should look next?