GM Bonneville/Eighty Eight/LeSabre 1986-1999

Bleeding the Brake System

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The purpose of bleeding the brakes is to expel air trapped in the hydraulic system. The system must be bled whenever the pedal feels spongy, indicating that compressible air has entered the system. It must also be bled whenever the system has been opened or repaired. You will need a helper for this job. You may need to bleed the hydraulic system at all 4 wheel positions if air has entered the system due to low fluid level, or from brake lines having been disconnected from the master cylinder. If a brake line is disconnected from one wheel, only that wheel caliper/cylinder needs to be bled. If lines are disconnected at any fitting located between the master cylinder and the wheels, then the brake system served by the disconnected line must be bled.


WARNING
For 487-90 ABS equipped vehicles, the procedure for bleeding the brake system is different than the procedure below. Please refer to the procedure for your vehicle, under the applicable ABS portion of this section. For 491-99 vehicles, the procedure is the same, regardless if the vehicle has ABS or not. However, for 494-99 vehicles with ABS, after bleeding the brake system you must perform an "Auto Bleed" sequence, using a Tech 1® or equivalent scan tool. If you do not have access to a suitable scan tool, your vehicle will probably have to be towed to a shop with the proper diagnostic equipment to have the procedure performed.

MANUAL BLEEDING



See Figures 1, 2, 3, 4, 5 and 6


CAUTION
Never reuse brake fluid which has been bled from the brake system. Brake fluid should be changed every few years. It wears out due to moisture being absorbed, which lowers the boiling point.

Old brake fluid is often the cause of spongy brakes returning a week or so after bleeding the system. If all parts check good. Change the fluid by repeated bleeding.

  1. Deplete the booster reserve by applying the brakes a few times with the engine OFF until all reserve is depleted.
  2.  
  3. Fill the master cylinder reservoir to the full level mark with brake fluid from a fresh, sealed container. You must maintain the fluid level during the bleeding procedure.
  4.  


WARNING
Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded. Also, do not allow any brake fluid to come in contact with a painted surface; it will damage the paint.

  1. If the master cylinder is known or suspected to have air in the bore, bleed it as follows before bleeding any of the wheel cylinders or calipers:
    1. Loosen the forward brake line connection and the master cylinder.
    2.  
    3. Fill the master cylinder until fluid begins to flow from the front pipe connector port, maintaining the fluid level.
    4.  
    5. Tighten the front brake pipe on the master cylinder securely.
    6.  

  2.  


WARNING
Make sure the master cylinder reservoir cap is installed before the brake pedal is pressed. This prevents brake fluid from spraying out of the reservoir.

    1. Have an assistant depress the brake pedal slowly one time, and hold. Loosen the forward brake pipe connection again to purge air from the bore. Tighten the connection and then slowly released the brake pedal. Wait 15 seconds. Repeat the sequence, including the 15 second wait, until all air is removed from the bore.
    2.  
    3. Tighten the brake pipe connection to 11 ft. lbs. (15 Nm).
    4.  

  1. After all the air has been removed from the front connection, repeat the same procedure at the rear connection of the master cylinder.
  2.  
  3. Individual wheel cylinders and calipers are bled only after all the air has been removed from the master cylinder.
  4.  


WARNING
Make sure the master cylinder reservoir cap is installed before the brake pedal is pressed. This prevents brake fluid from spraying out of the reservoir.

  1. If all of the wheel circuits must be bled, use one of the following two sequences, depending upon the year of your vehicle:
    1. 487-90 vehicles: right rear, left front, left rear, then right front.
    2.  
    3. 491-99 vehicles: right rear, left rear, right front, then left front.
    4.  

  2.  
  3. Fill the master cylinder reservoir with brake fluid. Keep the level at least 1 / 2 full during the bleeding operation.
  4.  
  5. Raise and support the vehicle safely.
  6.  
  7. Remove the bleeder valve cap, and place a proper size box end wrench, or suitable bleeder wrench, over the bleeder valve.
  8.  



Click image to see an enlarged view

Fig. Fig. 1: To bleed the rear brakes, remove the cap from the wheel cylinder bleeder screw (see arrow)



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Fig. Fig. 2: Attach a hose to the bleeder valve with the other end submerged in a container of clean brake fluid



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Fig. Fig. 3: If necessary, you can remove the bleeder valve from the wheel cylinder by unscrewing it

  1. Attach a transparent tube to the bleeder valve, and submerge the other end of the tube in a clear container, partially filled with clean brake fluid.
  2.  



Click image to see an enlarged view

Fig. Fig. 4: There are special bleeder wrenches available, that make bleeding the brakes easier

  1. Have an assistant slowly depress the brake pedal one time, and hold.
  2.  
  3. Loosen the bleeder valve to purge the air from the cylinder.
  4.  



Click image to see an enlarged view

Fig. Fig. 5: To bleed the front brakes, remove the protective cap covering the bleeder valve/screw



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Fig. Fig. 6: Attach the brake bleeder line, with the other end submerged in a container full of clean brake fluid, then open the valve

  1. Tighten the bleeder screw, then slowly release the brake pedal. Wait 15 seconds.
  2.  
  3. Repeat this sequence until all the air is removed. Depending upon the amount of air in the system, you may have to repeat the sequence at least 10 times to remove all of the air from the system.
  4.  
  5. Lower the vehicle and refill the master cylinder.
  6.  
  7. Check the brake pedal for sponginess. If the pedal is spongy, repeat the entire bleeding procedure.
  8.  
  9. For 494-99 vehicles, use a Tech 1® or equivalent scan tool to pressure bleed the brake system, using the "Auto Bleed" sequence. Then, repeat the wheel cylinder and caliper bleeding sequence.
  10.  

PRESSURE BLEEDING



See Figures 7 and 8

The pressure bleeding equipment you use MUST be of the diaphragm type. It must have a rubber diaphragm between the air supply and the brake fluid to prevent air, moisture, oil and all other contaminants from entering the system.

  1. Fill the master cylinder reservoir to the full mark with the proper type of brake fluid from a fresh, sealed container.
  2.  
  3. Install bleeder adapter tool J 29567, J 35589 or equivalent, to the master cylinder. The proper adapter must be installed on the master cylinder to avoid damaging the reservoir.
  4.  



Click image to see an enlarged view

Fig. Fig. 7: Pressure bleeding adapter used on 1987-90 vehicles



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Fig. Fig. 8: Later model vehicles use a different type of pressure bleeding adapter

  1. Charge the bleeder ball to 7-25 psi (140-172 kPa).
  2.  
  3. Connect the line/hose to the adapter, then open the line valve. The bleeder adapter has a bleed valve on top of the adapter to help eliminate air from the adapter.
  4.  
  5. Raise and safely support the vehicle.
  6.  
  7. Use one of the following two bleeding sequences, depending upon the year of your vehicle:
    1. 1987-90 vehicles: right rear, left front, left rear, then right front.
    2.  
    3. 1991-99 vehicles: right rear, left rear, right front, then left front.
    4.  

  8.  
  9. Remove the bleeder valve cap, and place a proper size box end wrench, or suitable bleeder wrench, over the bleeder valve.
  10.  
  11. Attach a transparent bleeder hose to the bleeder valve, and submerge the other end of the of the hose in a clean container partially filled with clean brake fluid.
  12.  
  13. Open the bleeder valve 1 / 2 to 3 / 4 of a turn, then allow the fluid to flow until no air is seen in the fluid.
  14.  
  15. Tighten the wheel cylinder bleeder valve to 88 inch lbs. (10 Nm) or the caliper bleeder valve to 115 inch lbs. (13 Nm).
  16.  
  17. Repeat the bleeding steps until all of the calipers/cylinders have been bled. Then, install the bleeder valve caps.
  18.  
  19. Carefully lower the vehicle. Check the brake pedal for sponginess. If the pedal is spongy, repeat the entire bleeding procedure.
  20.  
  21. Remove the bleeder ball and adapter.
  22.  
  23. Fill the master cylinder reservoir to the full mark.
  24.  
  25. For 1994-99 vehicles, use a Tech 1® or equivalent scan tool to pressure bleed the brake system, using the "Auto Bleed" sequence. Then, repeat the wheel cylinder and caliper bleeding sequence.
  26.  

BENCH BLEEDING THE MASTER CYLINDER




WARNING
All new master cylinders should be bench bled prior to installation. Bleeding a new master cylinder on the vehicle is not a good idea. With air trapped inside, the master cylinder piston may bottom in the bore and possibly cause internal damage.

  1. Remove the master cylinder from the vehicle, and plug the outlet ports.
  2.  
  3. Secure the master cylinder in a soft jawed bench vise, with the front end slightly down.
  4.  
  5. Remove the master cylinder reservoir cap.
  6.  
  7. Fill the master cylinder reservoir with clean, fresh brake fluid.
  8.  
  9. Use a blunt tipped rod (a long socket extension works well) to slowly depress the master cylinder primary piston about 1 in. (25mm). Make sure the piston travels full its full stroke. As air bleeds from the master cylinder, the primary piston will not travel the full inch.
  10.  
  11. Reposition the master cylinder in the vise with the front end tilted slightly up.
  12.  
  13. Stroke the primary piston about 1 in. (25mm) several times again.
  14.  
  15. Reposition the master cylinder in the vise, so it is level.
  16.  
  17. Loosen the plugs in the outlet ports on at a time. Then push the piston into the bore in order to force the air from the cylinder.
  18.  
  19. Tighten the plugs before allowing the piston to return to its original position. This prevents air from being drawn back into the cylinder.
  20.  
  21. Fill the master cylinder reservoir with clean brake fluid, then install the master cylinder reservoir cap.
  22.  
  23. Install the master cylinder on the vehicle, then follow normal bleeding procedures.
  24.  

 
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