Mac Haik @ I10 & Kirkwood replaced mine in early December. Bill Heard in Sugarland is also well aware of the problem. I do not know that GM has ever issued a TSB on the substandard vacuum boosters. Mac Haik had never heard of the problem until I insisted that they call the GM Tech Center. Talk to Harold in service. He should remember my QS Z06. Haik is a low allocation dealer so their Z customers should not be hard to remember.