It finally dawned on me what your problem probably is .
This post is from my work on Chrysler products with anti-lock brakes , Ford may do things differently, so I'll apologize in advance if this isn't your issue and the test doesn't work.
Anti-lock brake systems have a fluid pump and a pressure accumulator. ( to provide brake fluid under pressure , instantly , during the cycling of the anti lock brakes ) .
The accumulator pressure is built up by the electric pump when the key is on.
I believe what's happening in your case , is that what ever valving that is supposed to hold that pressure in the accumulator , is failing allowing the fluid back into the master cylinder reservoir.
To test if this might be your problem , drain some fluid from the reservoir , then with the ignition key OFF, pump the brake pedal about a dozen times.
Pumping the pedal should drain the accumulator pressure, and the brake fluid level should rise in the reservoir . Then you can wait and check the level in a while to see if the level is now stable.
If the fluid level is stable then you've found your problem , unfortunately , I don't believe it will be a cheap repair as the pump and accumulator will be sold as an assembly.