Ok let me start off by tell you any information I tell you is just general knowledge and is not model specific.
Normally when you service or repair brakes on a vehicle with 4 wheel anti lock brakes,its best to complete any and all steps necessary for that procedure found in a factory service manual.
this being said, servicing a problem with ABS brakes should be left to a trained technician. parts of a ABS system contain brake fluid under tremendous pressure and can kill you!! again this is a general Information.!!
When servicing brakes mechanical or hydraulic, even just checking the fluid level in the system.
You would have the key off and pump the brake pedal 25-30 times to release the pressure from the system this will raise the level of brake fluid in the reservoir to what is the true level of brake fluid.
Complete all repairs to the brake system replacing hoses and brakes shoes etc as needed make sure the rear brakes are properly adjusted.
whenever the system is drained out, or air gets in the system. like others have mentioned you may need to remove the master cylinder and bench bleed it using some adapters and clear tubing. once the master cylinder is free of air reinstall it reconnect to the lines.
With the master cylinder cover off and the fluid level topped off gravity bleed each wheel starting with the right rear first then left rear right front and left front last. make sure you have clean fresh fluid with no air from all four corners of the car.
Top off the master cylinder as needed during process and when done.
Close the cover. Open the rr bleeder have someone push down on the pedal to the floor and hold it, then close the bleeder screw. once your screw is closed have them release the brakes.
Repeat process until you do not see any air.
Once your satisfied your no longer getting any air and the system is closed,
turn the ignition key on for 10 seconds then start the car.
Push down on the brake pedal slowly 5-10 times. Shut the car off and see if the fluid level went down some in the Master cylinder.
If it did the pump has filled up the accumulator and the abs should work properly provided you cleared any codes and repaired any broken sensors or faulty wiring.
Some or all of this may not apply to your car, this is why it is extremely important to service your car with a factory manual by your side.