Brakes - Hard Pedal Feel/Low Spongy Pedal

GMC NUMBER: 88-T-129

GROUP: 5 - BRAKES

DATE: July, 1988

CORPORATE NUMBER: 865009R

SUBJECT:
IMPROVED BRAKE PEDAL FEEL

MODEL:
1988 C/K 10 MODEL TRUCKS

THIS BULLETIN IS DIVIDED INTO 2 DISTINCT SECTIONS IN ORDER TO ADDRESS SEPARATE CONDITIONS WHICH OCCUR ON DIFFERENT BRAKING SYSTEMS. BOTH CONDITIONS MAY BE DESCRIBED BY THE CUSTOMER AS BRAKE PEDAL FEEL.

SECTION 1

Improved Brake Pedal Feel JN3 Brake System:

Some customers may comment of hard brake pedal or lack of assist on some 1988 C/K trucks equipped with JN3 brakes (fourth digit of VIN is "D"). Beginning with the following VIN breakpoints, a new larger displacement brake vacuum booster will be on 1988 C/K 10 pickups with JN3 brake system. The new booster is being installed to improve power assist during braking and improve the "feel" of the brakes, especially for customers who operate their vehicle heavily loaded most of the time.

FT. Wayne - C1 GMC JZ 543800
K1 GMC JZ 544157

Oshawa - C1 GMC J1 564368
K1 GMC J1 564371

Pontiac - C1 GMC JE 528414
K1 GMC JE 528759

Customers with vehicles built before the VIN breakpoints who operate vehicles heavily loaded may prefer the improved brake pedal feel of the new booster. If so, the brake booster which is currently being used in production can be installed on vehicles with this condition. When installing the new booster, follow the procedure in Light Duty C/K Service Manual X-8731, Section 5A.

Option New Booster
Year Code Year Part Number

C/K 1 Under JN3 1988 18012114
6000 GVWR (Built before
with power the above listed
brakes VIN's)

For vehicles under warranty use:
Labor Operation: H1220 Labor Time: 1.0 hr.

SECTION 2

Improved Brake Pedal Feel - JN5 Brake System.

Some customers may comment of a "spongy" or soft brake pedal feel, or longer brake pedal travel on some 1988 C/K trucks equipped with JN5 brakes (fourth digit of VIN is "E") that were built in the Oshawa Assembly Plant between the VIN breakpoints stated below.

GMC vehicles build between J1545174 and J1552172.

In order to improve the brake pedal feel, the master cylinder and brake vacuum booster combination listed below may be installed.

Master Cylinder New Brake Vacuum Booster
P/N 18013977 P/N 18012114

Labor OP: T7660 Labor Time 1.0 hr.