When driving my 2000 pontiac sunfire, I will use my brakes and sometimes they go to the floor and the car barely comes to a stop. Sometimes they work fine though. If I pump the brakes they work fine as well. The times when the brake pedal goes to the floor, it feels as if not all the brakes are working. I have checked for leaks and found nothing.I have bled the brakes and there is lack a of pressure to some of the brakes. I believe it might have something to do with my ABS braking system.

Try this ABS bleeding procedure:

WITHOUT TECH 1® OR T-100®

Do not place your foot on the brake pedal through this entire procedure unless specifically instructed to do so.
This method can only be used if the ABS warning lamp is not illuminated and not DTC's are present.

1. Remove your foot from the brake pedal.
2. Start the engine and allow it to run for at least 10 seconds while observing the ABS warning lamp.
3. If the ABS warning lamp turned ON and stayed ON after about 10 seconds, the bleeding procedure must be stopped and a Tech 1® must be used to diagnose the ABS function.
4. If the ABS warning lamp turned ON for about 3 seconds, then turned OFF and stayed OFF, turn the ignition OFF .
5. Repeat Steps 1-4 one more time.
6. The entire brake system should now be bled by following the guide or pressure bleeding procedure.

1. Clean the master cylinder fluid reservoir cover and surrounding area, then remove the cover.
2. Add fluid, if necessary to obtain a proper fluid level, then put the reservoir cover back on.
3. Prime the ABS hydraulic modulator/master cylinder assembly as follows:
   1. Attach a bleeder hose to the rearward bleeder valve, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
   2. Slowly open the rearward bleeder valve.
   3. Depress and hold the brake pedal until the fluid begins to flow.
   4. Close the valve, then release the brake pedal.
   5. Repeat Steps 3b-3d until no air bubbles are present.
   6. Relocate the bleeder hose to the forward hydraulic modulator bleeder valve, then repeat Steps 3a-3e.
   
   
4. Once the fluid is seen to flow from both modulator bleeder valves, the ABS modulator/master cylinder assembly is sufficiently full of fluid. However, it may not be completely purged of air. At this point, move to the wheel brakes and bleed them. This ensures that the lowest points in the system are completely free of air and then the assembly can purged of any remaining air.

Fig. Fig. 3: ABS hydraulic modulator/master cylinder bleeder locations

1. Remove the fluid reservoir cover. Fill to the correct level, if necessary, then fasten the cover.
2. Raise and safely support the vehicle.
3. Proceed, as outlined in the following steps, to bleed the wheel brakes in the following sequence: right rear, left rear, right front, then left front.
   1. Attach a clear plastic bleeder hose to the bleeder valve at the wheel, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
   2. Open the bleeder valve.
   3. Have an assistant slowly depress the brake pedal.
   4. Close the valve and slowly release the release the brake pedal.
   5. Wait 5 seconds.
   6. Repeat Steps 7a-7e until the brake pedal feels firm at half travel and no air bubbles are observed in the bleeder hose. To assist in freeing the entrapped air, tap lightly on the caliper or braking plate to dislodge any trapped air bubbles.
   
   
4. Repeat Step 7 for the remaining brakes in the sequence given earlier.
5. Carefully lower the vehicle.
6. Remove the reservoir cover, then fill to the correct level with brake fluid and replace the cap.
7. Bleed the ABS hydraulic modulator/master cylinder assembly as follows:
   1. Attach a clear plastic bleeder hose to the rearward bleeder valve on the modulator, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
   2. Have an assistant depress the brake pedal with moderate force.
   3. Slowly open the rearward bleeder valve and allow the fluid to flow.
   4. Close the valve, then release the brake pedal.
   5. Wait 5 seconds.
   6. Repeat Steps 11a-11e until no air bubbles are present.
   7. Relocate the bleeder hose to the forward hydraulic modulator bleeder valve, then repeat Steps 11a-11f.
   
   
8. Carefully lower the vehicle, then check the brake fluid and add if necessary. Don't forget to put the reservoir cap back on.
9. With the ignition turned to the RUN position, apply the brake pedal with moderate force and hold it. Note the pedal travel and feel. If the pedal feels firm and constant and the pedal travel is not excessive, start the engine. With the engine running, recheck the pedal travel. If it's still firm and constant and pedal travel is not excessive, road test the vehicle and make sure the brakes are operating properly.
10. If the pedal feels soft or has excessive travel either initially or after the engine is started, the following procedure may be used:
    1. With the Tech 1® scan tool, Release then Apply each motor 2-3 times and cycle each solenoid 5-10 times. When finished, be sure to Apply the front and rear motors to ensure the pistons are in the upmost position. DO NOT DRIVE THE VEHICLE.
    2. If a Tech 1® scan tool is not available, remove your foot from the brake pedal, start the engine and allow it run for at least 10 seconds to initialize the ABS. DO NOT DRIVE THE VEHICLE. After 10 seconds, turn the ignition OFF . The initialization procedure most be repeated 5 times to ensure any trapped air has been dislodged.
    3. Repeat the bleeding procedure, starting with Step 1.
    
    
11. Road test the vehicle, and make sure the brakes are operating properly.