A very big high to you all. I have a 2004 F150 Lightning that I run as my daily driver. This is the 2nd Lightning I have owned, previously an 03 year model. My business partner also runs a 2004 Lightning.
The problem is that if you pump the brake pedal whilst the engine is running you can push the brake pedal to the floor. If you push real hard on the pedal she'll go to the floor. Without the engine running the pedal is hard. Now I did not have this problem on the 03 Lightning and my partners truck is fine.
I have replaced the brake master cylinder, the Servo unit and now the brake valve block and it is still the same. You can hear a hissing when you pump the brake pedal.
I have spoken to Ford SVT and they reckoned they had never come accross this problem before. Any help or advice would be greatly appreciated, thanks Graeme

 

the only thing i could think it would be is the vacuum booster, but if ford has already checked it, then there might be expansion in the rubber hoses, those would be the first thing to check.