1990 Chevrolet C3500 Pickup Brake Service (ONLINE REPAIR MANUALS)

BLEEDING EHCU MODULE

The EHCU Module should be bled after replacement or if trapped air is in the unit. If bleeding is required the module must be bled thoroughly before the wheel cylinders and calipers.

There are two bleeders on top of the unit that appear to be normal bleeders, Fig. . These are modulator bleeders and must remain closed when the unit is not pressurized.

The internal bleeders are on either side of EHCU module. The valves are used to open the internal passages within the EHCU module. Both bleed valves must be rotated ^1/4-^1/2 a turn counterclockwise before beginning t...

The Rear Wheel Anti-Lock (RWAL) system, Fig. , reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. Pressure applied to the rear wheel brakes is regulated by a control valve assembly located under the master cylinder. The control valve consists of a dump valve which releases hydraulic pressure into an accumulator, and an isolation valve which holds rear brake pressure. Operation of the dump valve and isolation valve is controlled by an Electronic Control Unit (ECU) which is mounted next to the master cylinder.

The ECU receives ...

CABALLERO & EL CAMINO

Pump brake pedal several times with engine off to deplete accumulator of fluid.

• Remove two nuts attaching master cylinder to booster, then move master cylinder away from booster with brake lines attached.
• Remove three hydraulic lines from booster. Plug and cap all lines and outlets.
• Remove retainer and washer securing booster pushrod to brake pedal arm.
• Remove four nuts attaching booster unit to dash panel.
• From engine compartment, loosen booster from dash panel and move booster pushrod inboard until it disconnects from brake pedal arm. Remove spring wa...

Diagnose power brakes systems as if two separate systems exist. Check for faults in the regular brake system first. If no problem exists in the regular brake system, diagnosis the power brake circuit is necessary. For a quick check of proper power unit operation, press the brake pedal firmly and then start the engine. The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position. On vacuum suspended power units, air will rush into the air intake when the brakes are applied.

To perform another check, remove the stop light switch, ...

WIRING DIAGRAMS

Refer to "Wiring Diagrams" in "Vehicle Systems."

READING TROUBLE CODES

Trouble codes are accessed by connecting a jumper wire between terminals A and H of the underdash ALDL connector and observing the flashes of the brake warning lamp. Note the following when performing the Diagnostic Circuit Check to access trouble codes.

• When the terminals are jumped, count the number of short flashes starting from the initial long flash and including the long flash in the count.
• The count sequence will repeat as long as the ALDL connector terminals are jumped with the ignition on.
• ...

The height sensing proportioning valve must be adjusted whenever the valve and/or linkage is disassembled or removed for service. In addition, adjustment should be checked if excessive front wheel lock-up is experienced with a lower than desired brake application rate when vehicle is at or near maximum GVWR.

• Raise and support vehicle leaving wheels on, and allow axle to hang free (no load condition).
• Remove lever retaining nut and disconnect lever from valve shaft, Fig. .
• Rotate valve shaft to permit installation of correct adjustment gauge, Fig. ,...