Brake System Bleeding
The content of this article reflects the changes called out by TSB 05-01-94.The base brake system must be bled anytime air is permitted to enter the hydraulic system. However, anti-lock brake system components should only be bled if the hydraulic control unit has been removed or if there is reason to believe that air has entered the unit. The ABS portion of the braking system must be bled separately. Under most circumstances, only the base brake system will require bleeding. If the anti-lock brake system must be bled, proceed as follows:
1. Bleed base brake system, then remove battery, battery tray, support bracket and acid shield.
2. Using suitable jumper cables, reconnect battery to vehicle battery cables, then connect DRB scan tool to diagnostic connector. Use extreme caution to avoid arcs and shorts against vehicle body when battery is connected through cables.
3. Using DRB, ensure Controller Antilock Brake (CAB) has no stored diagnostic trouble codes.
4. Bleed modulator primary check valve circuit, then the secondary check valve circuit as follows:
Fig. 242 Primary Check Valve Circuit Bleed Screw Location:
Fig. 243 Secondary Check Valve Circuit Bleed Screw Location:
a. Connect clear bleed tube to check valve circuit bleed screw, then place other end of tube in a clear glass container partially filled with clean brake fluid.
b. Pump brake pedal repeatedly, then depress pedal and exert medium to heavy pressure while loosening check valve circuit bleed screw one full turn.
c. When brake pedal bottoms, tighten bleed screw, then release pedal. Do not release pedal until bleed screw is tight.
d. Repeat procedure until brake fluid flowing through tube appears bubble-free and clean, then torque modulator assembly secondary bleed screw to 80 in. lbs. Do not remove bleed hose before screw is tight.
5. Bleed modulator primary sump circuit as follows:
Fig. 244 Primary Sump Circuit Bleed Screw Location:
a. Connect clear bleed tube to primary sump circuit bleed screw, then place other end of tube in a clear glass container partially filled with clean brake fluid.
b. Pump brake pedal repeatedly, then depress pedal and exert medium to heavy pressure while loosening primary sump circuit bleed screw one full turn.
c. Using DRB, select "Bleed ABS Hydraulic Unit" mode, then select "Primary Circuit." Right front and left rear solenoids will fire alternately for approximately five seconds.
d. Continue to select "Primary Circuit" on DRB until brake fluid flowing through bleed tube is bubble-free or until brake pedal bottoms.
e. If brake pedal bottoms before expelled fluid appears bubble-free, repeat procedure.
f. Torque primary sump bleed screw to 80 in. lbs., then release brake pedal and remove bleed tube from screw. Do not release pedal until bleed screw is tight.
6. Bleed modulator assembly primary accumulator circuit as follows:
Fig. 245 Primary Accumulator Circuit Bleed Screw Location:
a. Connect clear bleed tube to primary accumulator circuit bleed screw, then place other end of tube in a clear glass container partially filled with clean brake fluid.
b. Pump brake pedal repeatedly, then depress pedal and exert medium to heavy pressure.
c. Using DRB, select "Bleed ABS Hydraulic Unit" mode, then select "Primary Circuit Valves." Right front and left rear solenoids will fire alternately for approximately five seconds.
d. Loosen primary accumulator circuit bleed screw one full turn.
e. Continue bleeding primary accumulator circuit until brake fluid flowing through bleed tube is bubble-free or until brake pedal bottoms.
f. If brake pedal bottoms before expelled fluid appears bubble-free, repeat procedure.
g. Tighten primary accumulator bleed screw, then release brake pedal. Do not release pedal until bleed screw is tight.
h. Pump brake pedal repeatedly, then depress pedal and exert a constant heavy force while opening primary accumulator circuit bleed screw one full turn.
i. Without using DRB or allowing modulator solenoids to fire, bleed primary accumulator circuit until brake fluid flowing through tube appears bubble-free and clean.
j. When brake fluid appears as specified, torque primary accumulator bleed screw to 80 in. lbs. and release brake pedal. Do not release brake pedal until bleed screw is tight.
7. Bleed modulator assembly secondary sump circuit as follows:
Fig. 246 Secondary Sump Circuit Bleed Screw Location:
a. Connect clear bleed tube to secondary sump circuit bleed screw, then place other end of tube in a clear glass container partially filled with clean brake fluid.
b. Pump brake pedal repeatedly, then depress pedal and exert medium to heavy pressure while loosening secondary sump circuit bleed screw one full turn.
c. Using DRB, select "Bleed ABS Hydraulic Unit" mode, then select "Secondary Circuit Valves." Left front and right rear solenoids will fire alternately for approximately five seconds.
d. Continue to select "Secondary Circuit Valves" on DRB until brake fluid flowing through bleed tube is bubble-free or until brake pedal bottoms.
e. If brake pedal bottoms before expelled fluid appears bubble-free, repeat procedure.
f. Torque secondary sump bleed screw to 80 in. lbs., then release brake pedal and remove bleed tube from screw. Do not release pedal until bleed screw is tight.
8. Bleed modulator assembly secondary accumulator circuit as follows:
Fig. 247 Secondary Accumulator Circuit Bleed Screw Location:
a. Connect clear bleed tube to secondary accumulator circuit bleed screw, then place other end of tube in a clear glass container partially filled with clean brake fluid.
b. Pump brake pedal repeatedly, then depress pedal and exert medium to heavy pressure.
c. Using DRB, select "Bleed ABS Hydraulic Unit" mode, then select "Secondary Circuit Valves." Left front and right rear solenoids will fire alternately for approximately five seconds.
d. Loosen secondary accumulator circuit bleed screw one full turn.
e. Continue bleeding secondary accumulator circuit until brake fluid flowing through bleed tube is bubble-free or until brake pedal bottoms.
f. If brake pedal bottoms before expelled fluid appears bubble-free, repeat procedure.
g. Tighten secondary accumulator bleed screw, then release brake pedal. Do not release pedal until bleed screw is tight.
h. Pump brake pedal repeatedly, then depress pedal and exert a constant heavy force while opening secondary accumulator circuit bleed screw one full turn.
i. Without using DRB or allowing modulator solenoids to fire, bleed secondary accumulator circuit until brake fluid flowing through tube appears bubble-free and clean.
j. When brake fluid appears as specified, torque secondary accumulator bleed screw to 80 in. lbs. and release brake pedal. Do not release brake pedal until bleed screw is tight.
9. Remove bleed tube and container, then install battery acid shield, tray support bracket, tray and battery.