Your 1997 GMC is equipped with a Kelsey-Hayes 4WAL brk. sys.
The sequence Chris told you is correct but be aware that it may take up to 2qts. of fluid & needs to be ran through a [function test] to purge the air from the ABS unit.This can only be done with the use of a scan tool.(eg.OTC4000,TECH-1,TECH-2 or equivelent)