Master cylinder?
 

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. Im just reaching for stuff from what I have read but hoping someone with more knowledge could help.

You should NOT see any fluid on the back of the master cylinder (NEEDS TO BE REPLACED)
Dirty fluid. Can also cause the peddle to be spongy. moisture in the fluid. (BRAKE SYSTEM NEEDS TO BE FLUSHED)
Welcome to the forum

Brake booster should be checked.
 

Sponge like pedal if bled rite is the booster.check your pistons and wheel cylinders for leaks first.and for sure swap out old brake fluid.