GirlDriver,
I would put my money on the master cylinder.
Typically, if it were one of the wheel cylinders, you would probably see fluid on the back of a wheel, or on the brake backing plate.
You're probably not seeing any fluid on the ground, as you're pumping it out of the master cylinder while you're moving etc.
I would venture a guess that if you looked closely at the M/C, you'll either find fluid on it, or even the bottom of the cab, where the wind has blown it up along the bottom.
Fortunately, it's not that difficult of a job to replace, and bleed the brake system.
Of course I should be talking, my 52 has been sitting in the same spot since May, because of this very reason.
