Just signed up to help father in law. He has a 2003 F150 with the 5.4. His brakes get spongy sometimes. He has seen some fluid at the back of the cylinder and booster but not much or all the time. Ive read alot of posts and it could be a number of things. His fluid is dirty and he bleed the brakes months ago. That could point to the cap. I'm just reaching for stuff from what I have read but hoping someone with more knowledge could help.

 

Sounds like the master cylinder needs to be replaced. Dirt can get sucked into the system anywhere there is a leak in the system. There was some recall's on brake issues for the F150...you might want to run the VIN here and see if it might be one.

Spongy brakes are due to air, leaks or internal leaks like a bad ABS modulator.