As a rule, the brake circuits on most vehicles with anti-lock brakes can be bled in the usual manner, provided no air has gotten into the ABS modulator assembly. If the only components you replaced were downstream of the modulator (calipers, wheel cylinders, brake hoses or lines), chances are normal bleeding procedures will clear the lines of any unwanted air.

But what if you replaced the master cylinder, a brake line or valve ahead of the ABS modulator? Or, what if you had to replace the modulator or high pressure accumulator? Now you have a more difficult job ahead of you.

Air can be very difficult to remove from an ABS modulator assembly because of all the nooks and crannies inside the unit. The modulator may have eight to 10, or more, ABS/traction control solenoid valves, plus various check valves and dead-end ports. Some ABS modulators have special bleed screws to help you vent the trapped air when bleeding the system. Others do not and require the use of a scan tool to cycle the ABS solenoids while you bleed the system.