need to isolate the problem as being either mechanical or ABS.
- You need to scan the system for code(s). Does the ABS lamp stay on after initial start-up? If you haven't access to a scan tool, usually an auto chain store (AutoZone) will offer scanning.
- Jack the rear of the car up and rotate the tires by hand to see if they turn without about the same resistance. Apply the brakes. Rotate by hand again. Apply the handbrake and release. Does the right one now drag? Are the shoe(s) hanging on a worn spot on the backing plate not retracting fully? Does one brake cable not fully retract?
- Drive the car. Jack it up again and rotate the tires by hand. Is it dragging now where it wasn't when cold? Open the cylinder bleed screw and if excessive fluid pressure is released, you either have a blockage in that circuit (possible collapsed flexline) or the ABS pump may be frozen causing that one particular wheel to have constant pressure.
You can still bleed most systems with vacuum. If you do bleed an open circuit and air remains, you may have to energize the pump with a scan tool or there is a low cost tool available to energize most pumps to bleed it the pressure way.
ABS systems should have the fluid changed every two years at the least as if the pump is never energized, the static fluid will cause corrosion in the pump which will lead to pump failure. It is a good idea also to panic stop the vehicle in an open parking lot a few times a year to keep the brake fluid from becoming stagnet in the pump.