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

Combination Valve

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See Figure 1

The combination valve is located in the engine compartment, directly under the master cylinder. It is usually mounted to a bracket, which shares the master cylinder mounting studs on the brake booster. The valve consists of three sections: the metering valve, the warning switch and the proportioning valve.

The metering section limits the pressure to the front disc brakes until a predetermined front input pressure is reached. The specific pressure is enough to overcome the rear shoe retractor springs. Under 30 psi (200 kPa), there are no restrictions of the inlet pressures; this way pressures are allowed to equalize during the no brake period.



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Fig. Fig. 1: Cross-sectional view of a combination valve assembly

The proportioning section controls the outlet pressure to the rear brakes after a predetermined rear input pressure has been reached; this feature is provided for vehicles with light loads, to help prevent rear wheel lock-up. The by-pass feature of this valve assures full system pressure to the rear brakes in the event of a front brake system malfunction. Also, full front pressure is retained if the rear system malfunctions.

The pressure differential warning switch is designed to constantly compare the front and the rear brake pressures; if one circuit should malfunction, the warning light (on the dash) will turn on. The valve and switch are designed to lock in the warning position once the malfunction has occurred. The only way the light can be turned off is to repair the malfunction and apply a brake line force of 450 psi (3100 kPa).

REMOVAL & INSTALLATION



1994-95 Models

See Figure 2

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.



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Fig. Fig. 2: View of the combination valve mounting (RWAL)

  1. Disconnect the negative battery cable.
  2.  
  3. Disconnect the hydraulic lines from the combination valve one at a time. Immediately plug all openings to prevent system contamination or excessive fluid loss.
  4.  
  5. Unplug the electrical connector from the combination valve pressure switch.
  6.  
  7. If equipped with Rear Wheel Anti-Lock (RWAL) brakes, unplug the RWAL pressure valve connector.
  8.  
  9. On most vehicles, remove the master cylinder/combination valve-to-bracket retaining nuts, then remove the valve from the vehicle. On some early model vehicles the combination valve may be removed from the bracket by removing the retaining bolt. If desired, remove the bolt and valve leaving the bracket and master cylinder secured to the booster.
  10.  
  11. If used, remove the RWAL pressure valve.
  12.  

The combination valve is not repairable and must be replaced as a complete assembly.

To install:
  1. If applicable, install the RWAL pressure valve.
  2.  
  3. Install the combination valve and bracket assembly, then install and tighten the master cylinder/valve bracket retaining nuts to specification. For details, please refer to the master cylinder procedure located earlier in this section.
  4.  
  5. If used, attach the RWAL pressure valve connector.
  6.  
  7. Engage the pressure switch electrical connector.
  8.  
  9. Remove the caps (one at a time) and connect the brake lines to the combination valve, then tighten the fittings.
  10.  
  11. Check and refill the master cylinder reservoir, then properly bleed the hydraulic brake system.
  12.  

1996-97 Models

See Figure 3

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.



Click image to see an enlarged view

Fig. Fig. 3: View of the combination valve mounting (4WAL)

  1. Disconnect the hydraulic lines from the combination valve one at a time. Immediately plug all openings to prevent system contamination or excessive fluid loss.
  2.  
  3. Unplug the electrical connector from the combination valve pressure switch.
  4.  
  5. Unfasten the combination valve retaining nuts and remove the valve.
  6.  

To install:
  1. Install the combination valve and tighten the retaining nuts to 27 ft. lbs. (36 Nm).
  2.  
  3. Attach the combination valve pressure switch electrical connector.
  4.  
  5. Connect the hydraulic lines to the combination valve and tighten the fittings to 21 ft. lbs. (29 Nm).
  6.  
  7. Bleed the brake system as outlined later in this section.
  8.  

1998-99 Models

See Figure 4

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.

Prior to replacing the combination valve, note the identification code on the valve. You must have the proper code when replacing the valve so that you can replace it with the correct replacement part.

  1. Refer to the accompanying illustration when performing this procedure.
  2.  
  3. Disconnect the negative batter cable.
  4.  
  5. Unplug the valve electrical connection (10).
  6.  
  7. Disconnect the front and rear brake pipes from the valve. Immediately plug all openings to prevent system contamination or excessive fluid loss.
  8.  
  9. Unfasten the three Allen head bolts (11) which attach the combination valve to the (BPMV) Brake Pressure Modulator Valve (12).
  10.  
  11. Separate the combination valve from the BPMV (4).
  12.  
  13. Remove the two transfer tubes (13).
  14.  

Do not reuse the transfer tubes. Always install new ones.

To install:
  1. Check that new combination valve has the same code as the old one.
  2.  
  3. Install two new transfer tubes (13) into the new valve (12) until they are fully seated.
  4.  
  5. Attach the valve to the BPMV (4).
  6.  
  7. Install the Allen head bolts and tighten them to 6 ft. lbs. (8 Nm).
  8.  
  9. Remove the caps from the brake lines and attach them to the valve. Tighten the line fittings to 18 ft. lbs. (24 Nm).
  10.  
  11. Attach the valve electrical connection.
  12.  
  13. Connect the negative battery cable.
  14.  
  15. Bleed the brake system. Refer to the ABS brake bleeding procedure outlined later in this section.
  16.  



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Fig. Fig. 4: Exploded view of the combination valve mounting and its related components-1998-99 models

 
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