HEIGHT SENSING BRAKE PROPORTIONING VALVE ADJ.

SUBJECT: HEIGHT SENSING BRAKE PROPORTIONING VALVE ADJUSTMENT PROCEDURE

MODELS: 1984-85 C2 AND K2 SUBURBANS AND 1986-89 R2 AND V2 SUBURBANS

THIS BULLETIN CANCELS AND REPLACES GMC TRUCK SERVICE BULLETIN 86-B-93 DATED JUNE, 1986 (CORPORATE NUMBER 565001) TO ADD 1986-89 R2 AND V2 SUBURBANS AND TO UPDATE THE SERVICE INFORMATION. ALL COPIES OF 86-B-93 SHOULD BE DISCARDED.

The brake balance on 1984-1985 C2 and K2 Suburbans can be adjusted in order to enhance rear braking performance under lightly loaded vehicle conditions. 1986-89 R2 and V2 Suburbans were built with this new brake adjustment.

In order to change brake balance, it will be necessary to adjust the height sensing brake proportioning valve. In addition, burnishing the brakes following the procedure below may be beneficial on some vehicles. The following service procedure should also be used on all 1984-89 suburbans if the height sensing linkage has been disconnected from the valve during routine service.

IMPORTANT

For vehicles exhibiting a hard brake pedal feel with a cold engine or upon quick brake reapply, refer ence GMC Truck Service Bulletin 88-T-121 Dated June, 1988 (Corporate Number 865005).

SERVICE PROCEDURE

Note: Always make sure that the brakes are operational after performing brake work and before returning the vehicle to service.

- Raise vehicle, axle must be allowed to hang free (no load condition).

- Remove nut and disengage lever assembly from valve shaft (See Figure 1).

- Rotate valve shaft to permit installation of plastic adjustment gage. Center "D" shaped hole of gage must seat over "D" shape of the valve shaft and gage tang must be positioned in valve mounting hole (See Figure 2).

- Reinstall lever assembly to the valve shaft utilizing a "C" clamp or channel lock pliers to seat lever nylon bushing to serration on the valve shaft.

IMPORTANT

Do not drive the lever onto the shaft using the attaching nut. Doing so may result in improper valve setting.

- Reinstall valve shaft nut and torque to 8 - 11 Nm. (70 - 98 lbs. in.)

- Cut gage tang from body of the adjustment gage to allow the valve assembly to rotate freely (See Figure 3).

- Lower vehicle and test brakes.

SERVICE PARTS INFORMATION

Model P/N Code Color ------- -------- ---- ----- C2, R2 14061394 A Green K2, V2 15548904 E Yellow

Parts are currently available from GMSPO.

BRAKE BURNISHING

On dry clean level roadway, accelerate to 55 mph., observe traffic following and when its clear

Apply brake at a deceleration rate of 8 ft/sec/sec until vehicle speed is 25 mph. A decelerometer must be used to achieve the proper deceleration rate.

Repeat this procedure at 1 mile intervals 15 times.

Decelerometers are available from various sources, one of which is:

Ammco Tool Company 2100 Commonwealth Avenue N. Chicago, IL 60064

WARRANTY INFORMATION

For vehicles repaired under warranty use:

Labor Operation: H0777 Labor Time: .3 hr.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.