Disc & Drum Brake System: Other Chevrolet Pickup K2500

BRAKE WARNING LIGHT

Pressure differential warning switch in combination valve energizes brake warning light on instrument panel when front or rear brakes lose hydraulic pressure. After repairing failed side of hydraulic system, depress brake pedal with moderate to heavy pressure to hydraulically center the piston. This will turn off brake warning light.

CALIPERS (FRONT)

Front brakes are floating caliper or sliding caliper design. see scheme 22and see scheme 23. Rear brakes are sliding caliper design. Caliper is attached to caliper mount. Caliper is mounted to steering knuckle or caliper adapter, depending on application. Caliper assembly slides back and forth in machined cut-outs.

CALIPER (REAR)

The rear disc brakes use a Bendix caliper. The caliper mounts to the anchor plate in a way that allows the caliper to move laterally against the rotor. The caliper is a one piece casting with the inboard side containing a piston bore. A square-cut rubber seal fits in a groove in the piston bore to provide a hydraulic seal between the piston and caliper bore.

Combination Valve

System uses a combination valve to regulate brake system hydraulic pressure. Combination valve, located in brake lines between master cylinder and wheels, has 3 pressure control functions

1. Metering (or hold-off) section of valve limits pressure to front brakes until pressure of rear brake shoe retractor springs is overcome, then allows pressure to front brakes.
2. Proportioning section of valve allows input pressure to rise to predetermined level before allowing output pressure to rear brakes. This prevents rear wheel lock-up on vehicles with light rear wheel loads.
3. By-Pass section of valve ensures full system pressure is applied to rear brakes if front brakes loses hydraulic pressure (or if rear brakes lose hydraulic pressure, full pressure is applied to front brakes).

Combination valve also contains a pressure differential warning switch. See BRAKE WARNING LIGHT under DESCRIPTION & OPERATION .

Height-Sensing Proportioning Valve

Valve senses vehicle load conditions through relative movement between rear axle and body. (Scheme 33) As vehicle load increases (resulting in decreased vehicle height), valve allows higher pressure to rear brakes, resulting in optimum front-to-rear brake pressure balance.

Valves allow input pressure to rise to predetermined level before allowing output pressure to rear brakes. This prevents rear wheel lock-up on vehicles with light rear wheel loads. Valves also incorporate a by-pass feature which ensures full system pressure is applied to rear brakes if front brakes lose hydraulic pressure (or if rear brakes lose hydraulic pressure, full pressure is applied to front brakes).

ROTOR (REAR)

The rear disc brakes use a combination rotor and hub assembly. The rotor is integral with the rear hub. During operation, the rotor turns between the linings and basically free-wheels until the linings begin to apply a clamping action on it. The vented area between the rotor braking surfaces allows for efficient heat dissipation.

BLEEDING BRAKE SYSTEM

Note. On vehicles with ABS, wait 15 seconds between pedal strokes when bleeding brake system. On vehicles with Hydro-Boost, check for air bubbles in power steering fluid reservoir. If air bubbles are present, bleed Hydro-Boost system before bleeding brake system. See BLEEDING HYDRO-BOOST SYSTEM in POWER BRAKE BOOSTER - BENDIX HYDRO-BOOST article in this section.

BLEEDING SEQUENCE

BLEEDING SEQUENCE

MASTER CYLINDER BLEEDING

Note. To prevent air from entering brake system, bench bleed master cylinder before installing.

1. Place master cylinder in soft-jawed vise. DO NOT overtighten vise. Install bleeder tubes in both outlets of master cylinder. (Scheme 31) Fill master cylinder reservoir. Ensure ends of bleeder tubes are submerged in brake fluid.
2. Press and release piston assembly until air bubbles are no longer present in fluid flow. Leave bleeder tubes installed on master cylinder. Install master cylinder.
3. Remove bleeder tubes. Connect brake lines at master cylinder. DO NOT fully tighten brake lines at this time. Slowly press brake pedal to floor and hold. Tighten brake lines. Release brake pedal. Bleed brake system. See «MANUAL BLEEDING»(ref-46671-S21004106732001011500000), «PRESSURE BLEEDING»(ref-46671-S28659695132001011500000) or «VACUUM BLEEDING»(ref-46671-S39141170312001011500000).

Scheme 31

MANUAL BLEEDING

Note. Air tends to cling to caliper walls. When bleeding vehicles with disc brakes, lightly tap caliper to help remove air.

1. Fill master cylinder. Install bleeder hose to bleeder screw at first wheel to be serviced. See BLEEDING SEQUENCE under «BLEEDING BRAKE SYSTEM»(ref-46671-S23516664012001011500000) . Submerge other end of hose in glass jar partially filled with brake fluid.
2. Open bleeder screw 3/4 - 1 turn. Slowly press brake pedal to floor. Note fluid flow. Close bleeder screw. Release pedal. Repeat procedure until air bubbles are no longer present in fluid flow.

PRESSURE BLEEDING

Note. Air tends to cling to caliper walls. When bleeding vehicles with disc brakes, lightly tap caliper to help remove air.

1. Retain hold-off valve stem of combination valve using Valve Retainer (J-23709). (Scheme 32) This allows brake fluid to flow through combination valve and entire system during bleeding.
2. Clean master cylinder cap and surrounding area. Remove cap. With pressure tank at least 1/2 full, connect pressure bleeder to master cylinder with adapters. Attach bleeder hose to first bleeder valve to be serviced. See BLEEDING SEQUENCE under «BLEEDING BRAKE SYSTEM»(ref-46671-S23516664012001011500000).
3. Place other end of hose in glass jar partially filled with brake fluid so end of hose is submerged in fluid. Open release valve on pressure bleeder. Set pressure bleeder to 20-25 psi (1.4-1.8 kg/cm 2 ) or pressure specified by equipment manufacturer.
4. Open bleeder screw 3/4 - 1 turn and note fluid flow. Close bleeder screw when no air bubbles are present in fluid flow. Repeat procedure on remaining wheels in proper sequence.
5. Check brake pedal operation. Remove pressure bleeder. Remove valve retainer from hold-off valve. Refill master cylinder reservoir, if necessary.

Scheme 32

VACUUM BLEEDING

Note. Air tends to cling to caliper walls. When bleeding vehicles with disc brakes, lightly tap caliper to help remove air.

1. Fill master cylinder reservoir. Install vacuum bleed equipment to first bleeder valve to be serviced. See BLEEDING SEQUENCE under «BLEEDING BRAKE SYSTEM»(ref-46671-S23516664012001011500000) .
2. Open bleeder valve 3/4 - 1 turn. Press vacuum pump to draw fluid into reservoir jar until no air bubbles are present in fluid flow. Close bleeder screw. Repeat procedure on remaining wheels in sequence.

Note. If front wheel lock-up is experienced during braking when vehicle is near maximum GVWR, adjust height-sensing proportioning valve.

1. Ensure rear axle is hanging free and valve is securely bolted to frame when lever is attached to valve before height-sensing proportioning valve is adjusted. (Scheme 33)
2. Raise and support vehicle. Remove nut and lever assembly from valve. Rotate valve shaft to allow installation of plastic adjustment gauge No. GM 15592484. Seat "D" shaped center hole over valve shaft. Gauge must be positioned in valve mounting hole.
3. Install lever assembly to valve shaft with a "C" clamp or locking pliers to seat lever nylon bushing on serrated valve shaft. DO NOT install lever using attaching nut. Doing so can result in improper valve adjustment.
4. Install valve shaft nut and tighten to 80 INCH lbs. (9 N.m). Cut tang from body of adjustment gauge, and allow valve to rotate freely. Lower vehicle and test brakes.

Scheme 33

Vehicles With Leading/Trailing Brakes

1. Raise and support rear of vehicle. Remove rear wheels. Remove brake drums. Rotate parking brake adjusting screw until clearance between brake drum and shoes is .010-.020" (.25-.51 mm). Use brake drum diameter gauge to determine clearance.
2. Ensure stop on parking brake lever is in contact with edge of brake shoe web. If parking brake cable is holding stops off of shoe web, loosen cable adjusting nut. Tighten cable adjusting nut until lever stop begins to move off of shoe web.
3. Loosen cable adjusting nut until lever stop moves back toward lever stop and barely contacts the stop. Clearance should be no more than .020" (.51 mm) between lever stop and shoe web.
4. Install brake drums and wheels. Apply and release service brake pedal 30-35 times with normal pedal force, pausing about one second between pedal applications. Apply parking brake pedal 6 notches from fully released position.
5. Rear wheels should not be able to be rotated. Release parking brake and ensure wheels rotate freely. Lower vehicle.

Vehicles with Transmission-Mounted Parking Brake

1. Remove clevis pin from lever assembly. (Scheme 34) Apply parking brake pedal 4 notches from fully released position.
2. Using small length of cable or chain and a turnbuckle, install Spring Scale (J-35999) between lever assembly and vehicle frame. Tighten turnbuckle until spring scale indicates 50 lbs. (22.7 kg).
3. Loosen clevis nut (at cable end). Turn clevis until clevis pin slides freely into lever assembly with all slack removed from cable. Install clevis pin and retain with a cotter pin. Tighten clevis nut. Remove spring scale. Release parking brake.

Scheme 34

Vehicles With Dual-Servo Brakes

Raise and support vehicle. Loosen parking brake cable adjusting nut. Apply parking brake pedal 4 notches from fully released position. Tighten adjusting nut until wheels rotate forward with moderate drag. Release parking brake and ensure wheels rotate freely. Lower vehicle.

Brake shoe adjustment is accomplished by adjusting parking brake. See PARKING BRAKE under ADJUSTMENTS.

With Cruise Control

Depress and hold brake pedal. Pull lever on stoplight switch back to its stop. (Scheme 35) Pull brake pedal against pedal stop. Release brake pedal. Switch is adjusted if brake lights do not come on with pedal released.

Scheme 35

Electrical Circuit

Disconnect wire from switch terminal on combination valve. Connect wire to ground. Turn ignition on. If brake warning light does not come on, repair brake warning light bulb or wiring circuit. If light operates, brake warning light electrical circuit is okay.

Warning Light Switch

1. Fill master cylinder reservoir. Attach bleeder hose to bleeder screw at either rear wheel. Immerse other end of hose in container of brake fluid. Turn ignition on.
2. While depressing brake pedal, open bleeder screw (close bleeder screw before releasing pedal). On all models except Astro and Safari, if light comes on, go to step 3). If light does not come on, switch is defective; replace combination valve.
3. Close bleeder screw. Depress brake pedal with moderate to heavy pressure. If light goes out, switch is okay. If light stays on, switch is defective, replace combination valve. Repeat test on front brake system. System should function in same manner as rear.

REFINISHING BRAKE ROTORS (DISC BRAKES)

It is not necessary to refinish brake rotors when doing routine brake maintenance such as replacing brake shoe and lining assemblies. Refinish rotors only under the following circumstances

1. There is complaint of brake pulsation.
2. There are heat spots or excessive scoring.

Brake rotors have a minimum thickness cast into them. This dimension is the minimum wear dimension and not a refinish dimension. Do not use a brake rotor that will not meet the dimensions shown in the specifications chart after refinishing. A rotor that is too thin will not have the proper heart transfer capabilities. Replace it with a new rotor. Accurate control of rotor tolerances is necessary for the proper performance of disc brakes. Precision equipment should be used for machining. Service machining equipment on a regular basis. Always use sharp cutting tools or bits and use only replacement cutting bits recommended by the equipment manufacturer. Always use vibration dampening attachments when refinishing brake surfaces. They allow for a better surface finish.

Locate the deepest score and turn the rotor micrometer knobs until the tool bit bottoms out at the deepest point of the score. Zero the scale and back out the tool bits. Advance the cutter hand-wheel until the bits have cleared the inner edge of the rotor face. Adjust the micrometer knobs for approximately .005" (.0127 mm) more than the first reading. This will ensure clearing the rotor in one cut. It is very important that you make the rotor surface non-directional by dressing the rotor surfaces with a sanding disc power tool such as Ammco 8350 Safe Swirl Disc Rotor Grinder using 120 grit aluminum oxide sandpaper. Sand each rotor surface using moderate pressure for a minimum of 60 seconds. An alternate method is to use a sanding block with 150 grit aluminum oxide sandpaper. With the rotor turning approximately 150 RPM, sand each rotor surface using moderate pressure for a minimum of 60 seconds. After sanding the rotor clean with brake cleaner.

The finished rotor surface should be as close to that of a new rotor as possible. Failure to obtain the best possible rotor finish can affect initial braking performance.

Scheme 36

Scheme 37

Rotor Tolerance & Surface Finish

Tolerances of the braking surfaces for flatness, parallelism and lateral runout are closely held. The maintenance of close tolerances on the shape of the breaking surfaces is necessary to prevent brake roughness or pulsation. The surface finish must be held to a specific range of 60 Ra roughness or less. The control of the surface can improve lining life. Light scoring of the rotor surfaces not exceeding 1.5 mm (0.06 in.) in depth is normal and not detrimental to brake operation.

Thickness Variation Check

Check thickness variation by measuring the rotor thickness at four or more points around the circumference of the rotor. Use a micrometer calibrated in ten-thousands of an inch. Make all measurements at the same distance in from the edge of the rotor. A rotor measuring thicker than .0005" (.013 mm) can cause pedal pulsation and/or front end vibration during brake applications. A rotor like this should be refinished or replaced.

Lateral Runout Check

The best way to check lateral runout is with the wheels still installed on the vehicle. This gives a much more accurate reading of the total indicated runout under real braking conditions. If equipment is not available to perform the check with the wheels installed, the next best reading can be made with the wheels removed but the caliper still installed.

1. Clean rotor surface. If the wheel must be removed, reinstall the wheel nuts to retain the rotor. Tighten the wheel nuts to the correct torque specification following the wheel nut tightening sequence.
2. Fasten a dial indicator to the steering knuckle so the indicator button contacts the rotor surface about .5" (13 mm) from the outer edge.
3. Set the dial indicator to zero.
4. Turn the wheel one complete revolution and observe the runout indicated on the dial.
5. The total indicated runout must not exceed .0003" (.08 mm).
6. If lateral runout is not within specifications, refinish or replace the rotor as necessary. In some cases, excessive lateral runout can be improved by indexing the rotor on the hub one or two bolt positions from the original position. If lateral runout cannot be corrected by indexing the rotor, check the hub and bearing assembly for excessive lateral runout or looseness. If the hub and bearing assembly lateral runout exceeds .0015" (.040 mm), repair or replace the hub and bearing assembly. NOTE: Any time the brake rotor has been separated from the wheel bearing flange, clean any rust or foreign material from the mating surfaces of the wheel bearing flange and rotor. Failure to do so can result in increased lateral runout and brake pulsation.

BURNISHING LININGS & ROTORS (DISC BRAKES)

After the brake linings are replaced and/or the rotors refinished, the new braking surface should be broken in, or "burnished". To do this, make 20 stops from 30 mph using medium to firm brake pedal pressure. During this procedure, use care to avoid overheating the brakes.

SYSTEM BLEEDING

1. Place transmission in Park to set control valve in Park mode. Start engine. Turn steering wheel from stop to stop to bleed power steering system. Place transmission in Neutral.
2. Open bleed screw on actuator until fluid flow is free of air, then close bleed screw (bleed screw is near fluid hose connection, on top of actuator). Loosen exhaust line fitting (port EC) at control valve. (Scheme 31) Place transmission in Park. Allow small amount of fluid to bleed from fitting, then tighten fitting.
3. Cycle the system. If noisy, repeat bleeding procedure. Check power steering fluid level with transmission in Neutral and parking brake applied manually. Fluid level should be 7.12" (180.9 mm) from top of reservoir.

FRONT BRAKE CALIPER

Note. For front disc pad removal and installation, perform FRONT BRAKE CALIPER removal and installation procedures but do not disconnect brake hose from caliper (hang caliper out of way with wire). Replace all pads on an axle if wear indicator on any pad contacts rotor or if pad is worn to within 1/32" (.8 mm) of pad backing.

REAR BRAKE CALIPER

Note. For rear disc pad removal and installation, perform REAR BRAKE CALIPER removal and installation procedures but DO NOT disconnect brake hose from caliper (hang caliper out of way with wire). Replace all pads on an axle if wear indicator on any pad contacts rotor or if pad is worn to within 1/32" (.8 mm) of pad backing.

REAR BRAKE SHOES

Note. For rear brake shoe removal and installation see scheme 24 DO NOT interchange left and right adjusting screw assemblies as one side is right-hand thread and other is left-hand thread.

POWER BRAKE BOOSTER

Note. For Hydro-Boost removal and installation, see POWER BRAKE BOOSTER - BENDIX HYDRO BOOST article in the BRAKES Section.

REAR AXLE HUB BEARINGS & OIL SEAL (FULL-FLOATING AXLE)

Note. Outer bearing race is seated against a retaining ring inside hub, on inboard side of race. Remove retaining ring from inside of hub before driving out race.