I just calculated break percentages for agassi and chang vs sampras from 91-2000
agassi vs sampras : 24 matches, 57 breaks,333 return games , return games won % = 17.117
chang vs sampras : 11 matches, 15 breaks, 144 return games, return games won %= 10.41667
( atp website doesn't have stats for their 2 grand slam cup matches )
its not even close vs sampras !