If your 2004 Ford Focus has a manual transmission, the clutch has its own master cylinder that uses the same brake fluid as the brake master cylinder. If you removed any components on the clutch, you need to bleed the clutch master cylinder of air once you've reinstalled the parts. Once you locate the clutch master cylinder, bleeding it is very similar to bleeding a brake caliper.
Remove the car's air cleaner and intake duct--loosen the duct's clip with a screwdriver, disconnect the airflow sensor wiring, pull out the hose from the valve cover, unscrew and remove the air-cleaner cover and lift the cleaner off the rubber grommets.
Disconnect the battery--loosen the clamp for the negative cable with a wrench, followed by the positive one, then unbolt the battery's hold-down strap and lift the battery out. Unscrew and remove the battery tray.
Move any other pipes and wiring out of the way so you can access the clutch master cylinder.
Unscrew and remove the cap for the bleeder screw, located along the side of the master cylinder.
Connect a length of plastic or rubber tubing to the bleeder screw and place the tube's other end in a container of brake fluid. Hold the container about a foot above the bleeder screw to help maintain pressure in the system.
Open the bleeder screw by a turn or two and have an assistant press down on the clutch pedal; look for air bubbles to come out with the fluid through the tube. Tighten the screw once your assistant has completely depressed the pedal and have him release the pedal.
Repeat the above step until no air bubbles come out of the tube, then tighten the bleeder screw all the way with the clutch pedal completely depressed.
Open the filler cap for the brake master cylinder reservoir and add fresh brake fluid as needed.
Reconnect the battery tray with its screws and then strap the battery in place (tighten the strap bolt with the wrench). Connect the positive battery cable and then the negative one.
Install the air cleaner and intake duct, screwing the cover in place with the screwdriver, connecting the hose to the valve cover and the airflow sensor wiring, and connecting the intake duct to the airflow sensor.