Brake problem

[marinaman480]

I have just replaced the brake pads and shoes and rear wheel cylinders and bled the entire system and got a nice firm pedal however when i start the engine the pedal goes soft at the top of the travel and you dont have confidence in braking but when you switch of and pump the pedal goes nice and firm again, and whilst having foot on pedal and starting the engine the pedal goes down and feels soft again so i assume its to do with the servo any ideas ??????