GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

Bleeding the Hydraulic Brake System

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Models not equipped with Anti-lock Brake system (abs)

A scan tool is absolutely necessary to bleed the brake hydraulic system on models equipped with an Anti-lock Brake System (ABS). If your vehicle is equipped with ABS, be sure to refer to the ABS bleeding procedures in this section before performing any work on your vehicle's brake hydraulic system.

This brake bleeding procedure is for models that are not equipped with ABS. If your vehicle is equipped with an ABS system, refer to the ABS bleeding procedure later in this section.

The hydraulic brake system must be bled any time one of the lines is disconnected or any time air enters the system. If a point in the system, such as a wheel cylinder or caliper brake line is the only point which was opened, the bleeder screws down stream in the hydraulic system are the only ones which must be bled. If however, the master cylinder fittings are opened, or if the reservoir level drops sufficiently that air is drawn into the system, air must be bled from the entire hydraulic system. If the brake pedal feels spongy upon application, and goes almost to the floor but regains height when pumped, air has entered the system. It must be bled out. If no fittings were recently opened for service, check for leaks that would have allowed the entry of air and repair them before attempting to bleed the system.

As a general rule, once the master cylinder is bled, the remainder of the hydraulic system should be bled starting at the furthest wheel from the master cylinder and working towards the nearest wheel. Therefore, the correct bleeding sequence is: master cylinder, , right rear wheel cylinder, left rear, right front caliper and left front. Most master cylinder assemblies on these vehicles are NOT equipped with bleeder valves, therefore air must be bled from the cylinders using the front brake pipe connections.

Manual Bleeding

See Figures 1 and 2



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Fig. Fig. 1: Position a bleeder wrench over the wheel cylinder bleeder screw (rear wheels)



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Fig. Fig. 2: Attach a hose to the screw and submerge the other end in a container of clean brake fluid

For those of us who are not fortunate enough to have access to a power bleeding, the manual brake bleeding procedure will quite adequately remove air from the hydraulic system. The major difference between the pressure and manual bleeding procedures is that the manual method takes more time and will require help from an assistant. One person must depress the brake pedal, while another opens and closes the bleeder screws.

In addition to a length of clear neoprene bleeder hose, bleeder wrenches and a clear bleeder bottle (old glass jar or drink bottle will suffice), bleeding late-model ABS systems may also require the use of one or more relatively inexpensive combination valve pressure bleeding tools (which are used to depress one or more valves in order to allow component/system bleeding). To fully bleed the late model ABS systems, a scan tool should also be used to run the system through functional tests.

  1. Clean the top of the master cylinder, remove the cover and fill the reservoirs with clean fluid. To prevent squirting fluid, and possibly damaging painted surfaces, install the cover during the procedure, but be sure to frequently check and top off the reservoirs with fresh fluid.
  2.  


WARNING
Never reuse brake fluid, which has been bled from the system.


WARNING
Do not allow brake fluid to spill on or come in contact with the vehicle's finish as it will remove the paint. In case of a spill, immediately flush the area with water.

  1. The master cylinder must be bled first if it is suspected to contain air. If the master cylinder was removed and bench bled before installation it must still be bled, but it should take less time and effort. Bleed the master cylinder as follows:
    1. Position a container under the master cylinder to catch the brake fluid.
    2.  
    3. Loosen the front brake line at the master cylinder and allow the fluid to flow from the front port.
    4.  
    5. Have a friend depress the brake pedal slowly and hold (air and/or fluid should be expelled from the loose fitting). Tighten the line, then release the brake pedal and wait 15 seconds. Loosen the fitting and repeat until all air is removed from the master cylinder bore.
    6.  
    7. When finished, tighten the line fitting to 20 ft. lbs. (27 Nm).
    8.  
    9. Repeat the sequence at the master cylinder rear pipe fitting.
    10.  

  2.  

During the bleeding procedure, make sure your assistant does NOT release the brake pedal while a fitting is loosened or while a bleeder screw is opening. Air will be drawn back into the system.

  1. Check and refill the master cylinder reservoir.
  2.  

Remember, if the reservoir is allowed to empty of fluid during the procedure, air will be drawn into the system and bleeding procedure must be restarted at the master cylinder assembly.

  1. On late model ABS equipped vehicles, perform the special ABS procedures as described later in this section. On 4 wheel ABS systems the Brake Pressure Modulator Valve (BPMV) must be bled (if it has been replaced or if it is suspected to contain air) and on most Rear Wheel Anti-Lock (RWAL) systems the combination valve must be held open. In both cases, special combination valve depressor tools should be used during bleeding and a scan tool must be used for ABS function tests.
  2.  
  3. If a single line or fitting was the only hydraulic line disconnected, then only the caliper(s) or wheel cylinder(s) affected by that line must be bled. If the master cylinder required bleeding, then all calipers and wheel cylinders must be bled in the proper sequence:
    1. Right rear
    2.  
    3. Left rear
    4.  
    5. Right front
    6.  
    7. Left front
    8.  

  4.  
  5. Bleed the individual calipers or wheel cylinders as follows:
    1. Place a suitable wrench over the bleeder screw and attach a clear plastic hose over the screw end. Be sure the hose is seated snugly on the screw or you may be squirted with brake fluid.
    2.  

  6.  

Be very careful when bleeding wheel cylinders and brake calipers. The bleeder screws often rust in position and may easily break off if forced. Installing a new bleeder screw will often require removal of the component and may include overhaul or replacement of the wheel cylinder/caliper. To help prevent the possibility of breaking a bleeder screw, spray it with some penetrating oil before attempting to loosen it.

    1. Submerge the other end of the tube in a transparent container of clean brake fluid.
    2.  
    3. Loosen the bleed screw, then have a friend apply the brake pedal slowly and hold. Tighten the bleed screw to 62 inch lbs. (7 Nm), release the brake pedal and wait 15 seconds. Repeat the sequence (including the 15 second pause) until all air is expelled from the caliper or cylinder.
    4.  
    5. Tighten the bleed screw to 62 inch lbs. (7 Nm) when finished.
    6.  


  1. Check the pedal for a hard feeling with the engine not running. If the pedal is soft, repeat the bleeding procedure until a firm pedal is obtained.
  2.  
  3. If the brake warning light is on, depress the brake pedal firmly. If there is no air in the system, the light will go out.
  4.  
  5. After bleeding, make sure that a firm pedal is achieved before attempting to move the vehicle.
  6.  

Pressure Bleeding

See Figures 3 and 4

For the lucky ones with access to a pressure bleeding tool, this procedure may be used to quickly and efficiently remove air from the brake system. This procedure may be used as guide, but be careful to follow the tool manufacturer's directions closely. Any pressure bleeding tool MUST be of the diaphragm-type. A proper pressure bleeder tool will utilize a rubber diaphragm between the air source and brake fluid in order to prevent air, moisture oil and other contaminants from entering the hydraulic system.



Click image to see an enlarged view

Fig. Fig. 3: Pressure bleeding tool

  1. Prepare a pressure bleeder tool such as J-29567 or equivalent by making sure the pressure tank is at least 2 / 3 full of fresh, clean brake fluid. In most cases, the bleeder must be bled each time fluid is added. Charge the bleeder tool to 20-25 psi (140-170 kPa).
  2.  
  3. Install a suitable combination valve depressor tool such J-39177 or equivalent, to the combination valve in order to hold the valve open during the bleeding operation.
  4.  
  5. Install the pressure bleeder tool to the master cylinder reservoir.
  6.  
  7. On 4 wheel ABS systems, refer to the ABS bleeding procedures later in this section to bleed the Brake Pressure Modulator Valve (BPMV) of air.
  8.  



Click image to see an enlarged view

Fig. Fig. 4: Depressing the combination valve

  1. Bleed each wheel cylinder or caliper in the proper sequence:
    1. Right rear
    2.  
    3. Left rear
    4.  
    5. Right front
    6.  
    7. Left front
    8.  

  2.  
  3. Connect a hose from the bleeder tank to the adapter at the master cylinder, then open the tank valve.
  4.  
  5. Attach a clear vinyl hose to the brake bleeder screw, then immerse the opposite end into a container partially filled with clean brake fluid.
  6.  
  7. Open the bleeder screw 3 / 4 turn and allow the fluid to flow until no air bubbles are seen in the fluid, then close the bleeder screw and tighten to 62 inch lbs. (7 Nm).
  8.  
  9. Repeat the bleeding process at each wheel.
  10.  
  11. Inspect the brake pedal for sponginess and if necessary, repeat the entire bleeding procedure.
  12.  
  13. Remove the depressor tool from the combination valve and the bleeder adapter from the master cylinder.
  14.  
  15. Refill the master cylinder to the proper level with brake fluid.
  16.  
  17. DO NOT attempt to move the vehicle unless a firm brake pedal is obtained.
  18.  

 
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