Ford does not have a maintenance interval for changing the brake fluid. The European automakers specify changing the fluid every two years regardless of milage, but they use DOT 4 fluid that is more prone to absorbing moisture.
I change mine every year. I also use DOT 4 fluid (usually Valvoline synthetic).
I personally believe that DOT 3 fluid should be changed at least every four years.
The reason for changing the fluid is that all brake fluid, even in a sealed system, will take on moisture. Changes in pressure as the system is used as well as changes in temperature will ever so slightly compromise the ability to keep atmospheric moisture and air out. The miosture will degrade the fluid. The boiling point of the fluid drops as it becomes "wet" (that is that the fluid has taken on moisture). The specs on the brake fluid container usually give the boiling points for both dry (new) and wet (fluid with moisture in it). Also the moisture will react with the metal in the system and start to cause corosion and pitting. The fluid eventually becomes gunky and caustic. This contaminated fluid will contaminate and plug up the small holes in the ABS controller as well as cause calipers, wheel cylinders, and master cylinders to stick and bind.
It does save time to remove whatever fluid you can easily suck out of the master cylinder resivioir. In doing so, you will not get air into the working parts so there is no danger of causing problems from introducing air into the system. When you have sucked out as much as you can, top off the resivoir with fresh fluid and proceed with bleeding the system at each of the four wheels.
There are at least four methods of bleeding; pressure, gravity, vacuum, and pedal pumping.
Pressure bleeding requires the use of a system to pressurize the resivoir and then opening the bleed port on each wheel individually until there is no air bubbles coming out with the fluid and the fluid is as pristine as the new fluid. This is not done much any more and the pressure tools are not as common as they were in years past.
Gravity bleeding is to open the bleed port on the caliper or wheel cylinder and allowing it to just drip for a half hour or so. Often all four ports are left open at the same time. Care must be taken to not allow the resivoir to run dry. This method sometimes works when all other methods still seem to have left some air behind. Some technicians use this method frequently, and others use it only on hard cases.
Vacuum bleeding requirs the use of some sort of vacuum source and a catch bottle. There is a shop tool that uses compressed air to create a high vacuum to draw the fluid into a catch bottle and it has become very popular today. A home mechanic can use a MityVac pump with the brake bleeding attachments and accomplish the same thing. I use one of these at home. With the vacuum system, you attach a hose to the bleed port, apply vacuum, and open the bleed port. There will always be some air in the hose because the vacuum will pull air past the bleeder screw threads. Basically you suck out fluid until the fluid coming out is as pristine as the new fluid. Repeat for each wheel.
The pumping the pedal method has stood the test of time. It is best done with an assistant who will pump the pedal for you as you open the bleed port. Have the assistant pump the pedal until it is firm, then hold it steady. While they are holding the pedal steady, open the bleed port. Close the bleed port when the pedal drops to the end of it's stroke and repeat the process until the fluid is as pristine as the new fluid and there is no air coming out with the fluid. Repeat for each wheel.
With any of these methods, take care not to let the resivoir run out of fluid. Usually check and add fluid after two wheels have been completed.
If you have a car with hydraulic clutch linkage (like out MTX cars), you should also bleed the clutch using the same techniques.
I hope this helps.
