Brake System Bleeding GMC Pickup R3500

DESCRIPTION

Hydraulic system bleeding is necessary any time air has been introduced into system. Bleed brakes at all 4 wheels if master cylinder lines have been disconnected or master cylinder has run dry. Bleed brakes with vacuum bleeding equipment, pressure bleeding equipment or by manually pumping brake pedal while using bleeder tubes. Always bleed brake lines in sequence. See BLEEDING SEQUENCE table in this article.

Note. Hydro-Boost bleeding procedure is different than hydraulic brake bleeding. See BRAKE SYSTEM article.

Scheme 1

1. Master cylinder must be bled before installation to prevent excessive amounts of air from entering the brake system, creating poor brake operation.
2. Place master cylinder in soft-jawed vise. DO NOT tighten vise enough to damage master cylinder. Install bleeder tubes in both outlets of master cylinder. (Scheme 1)
3. Fill master cylinder with clean DOT 3 brake fluid. Ensure ends of bleeder tubes are submerged in brake fluid.
4. Using proper sized rod, apply and release master cylinder until no air bubbles exist in brake fluid flow. Once all air bubbles are gone from master cylinder secure cap and install.
5. Bleeder tubes should be left installed on master cylinder until master cylinder in installed. Master cylinder must be bled at wheels after installation.

Bleeding On Vehicle

1. Install master cylinder on vehicle after bench bleeding. Remove bleeder lines and install brake lines. DO NOT fully tighten brake lines at this time.
2. Slowly force brake pedal to the floor and hold in this position. Tighten brake lines and release brake pedal. Repeat procedure until no air bubbles exist at brake lines. Remaining wheel cylinder or calipers may require bleeding.

Hold Off Valve

1. Prior to pressure tank bleeding procedure, the hold off valve incorporated in the combination valve must be correctly positioned. This allows brake fluid to flow through combination valve to entire brake system.
2. During bleeding procedure, valve stem of hold off valve may be retained using Valve Retainer (J-23709). (Scheme 2) Remove valve retainer once brake bleeding procedure is complete.

Scheme 2

Note. On vehicles equipped with Hydro-Boost, ensure no air bubbles exist in power steering reservoir. If air bubbles exist, Hydro-Boost system must be bled before bleeding brake system. See BLEEDING, HYDRO-BOOST SYSTEM under HYDRAULIC POWER BRAKE UNIT in the BRAKE SYSTEM article.

VACUUM BLEEDING

Fill master cylinder. Install vacuum bleed equipment to first bleeder valve to be serviced. Open bleeder valve 3/4-1 turn. Depress vacuum pump and pull fluid into reservoir jar. Bleed each bleeder valve in proper sequence. See BLEEDING SEQUENCE table.

PRESSURE BLEEDING

1. Clean master cylinder cap and surrounding area. Remove cap. With pressure tank at least 1/2 full, connect to master cylinder with adapters. Attach bleeder hose to first bleeder valve to be serviced. See «BLEEDING SEQUENCE»(/gmc/pickup-r3500/1987-1991/remont/mechanical-hydraulic/#brake-system-bleeding) table.
2. Place other end of hose in clean glass jar partially filled with clean brake fluid so end of hose is submerged in fluid. The hold off valve must be positioned properly before pressure bleeding (if equipped). See «HYDRAULIC CONTROL VALVES»(/gmc/pickup-r3500/1987-1991/remont/mechanical-hydraulic/#brake-system-bleeding) .
3. Open release valve on pressure bleeder. Set pressure bleeder so 20-25 psi. (1.4-1.8 kg/cm 2 ) is used for bleeding pressure or follow equipment manufacturer's pressure instructions. Open bleeder screw 3/4-1 turn and note fluid flow.
4. Close bleeder screw when fluid flowing is free of bubbles. Repeat procedure on remaining wheels in proper sequence. Check brake pedal operation after bleeding has been completed.
5. Remove pressure bleeding equipment and valve retainer from hold off valve. Ensure that master cylinder is full of fluid.

MANUAL BLEEDING

Note. When bleeding disc brakes, air may tend to cling to caliper walls. Lightly tap caliper, while bleeding, to aid in removal of air.

1. Fill master cylinder. Install bleeder hose to first bleeder valve to be serviced. See «BLEEDING SEQUENCE»(/gmc/pickup-r3500/1987-1991/remont/mechanical-hydraulic/#brake-system-bleeding) table. Submerge other end of hose in clean glass jar partially filled with clean brake fluid.
2. Open bleeder valve 3/4-1 turn. Depress brake pedal slowly through full travel. Close bleeder valve and release pedal. Repeat procedure until flow of fluid shows no signs of air bubbles.

Note. When bleeding brake system manually, ensure bleeder valve is closed when brake pedal is released.

BLEEDING SEQUENCE

Before bleeding system, exhaust all vacuum from power unit by depressing brake pedal several times. Bleed master cylinder with bleeder screws. Bleed slave cylinder on vehicles equipped with remote mount power assist units. Bleed wheel cylinders and calipers in sequence. See BLEEDING SEQUENCE table.

BLEEDING SEQUENCE

See also:
• BRAKE SYSTEM
• BLEEDING SEQUENCE