Have you ever changed the brake pads? With 70,000 miles, they may be about worn out, depending on how much you torture the brakes. As the brake pads wear, they get thinner. As they get thinner, the caliper pistons move farther and farther out of the caliper. Check your brake pads to see how close the pad backing plates are to the rotors - if you have almost no friction material left, you've just figured out where your brake fluid went - it's still in the system, but it has migrated from the master cylinder to the caliper piston cylinders and you haven't actually lost any fluid. When new pads are installed, you typically have to crack open the bleeder screw when you push the caliper pistons into the calipers - this lets all the extra fluid out. If you don't do that, it pushes all the excess fluid back into the master cylinder and may overflow the master cylinder (not to mention is pushes dirty old brake fluid back into the brake system).
If the master cylinder level got so low that you got air in the system, you'll have a fun time bleeding the system. You might need to bleed the master cylinder as well to get all the air out. With the added complexity of the anti-lock braking system, it makes it that much more difficult to get all the air out.
2003 F-250 4x4, 7.3L PowerStroke
6" Skyjacker Z-link lift, 35" MT Baja ATZ tires
Warn 16.5Ti winch w/ hidden mount
Front & Rear ARB Air Lockers
Dynatrac Free-Spin Front Hub Conversion
Edge Juice Programmer
Fabtech Ladder Bars
AutoEnginuity w/ Ford Enhanced package - Southeastern Connecticut - Just bring beer!