Yep been doing "Brake Jobs" started with my Dad for >70 years! Those were drum brakes, back in the day. Had many performance cars including 3 of my how 7 Vettes since my 1st a 1988. All had "performance suspension" including brakes. My 2008 C8 Z51 brakes did NOT need burnishing (as GM calles it, suggest Googling Bedding the more common used word as lots of info why it's important) and why it is recommended by GM for all performance brakes on Track Capable Vettes. My 1st issue was with the Brembo brakes on my 2014 C7 Z51 at 1000 miles. I stopped with 5/6 hard braking allowing a minute between, followed by 10+ minutes of allowing the brakes to cool to room temp WITHOUT EVER STOPPING!

It does not tell you the common cause of squealing (a bit to technical for an Owner's Manual) which is an uneven microscopic layer of pad material that should be uniformly distributed on the rotor OR you'll get Slip/Stick friction (can google that if you'd like. But basically squealing is cause by vibration, could be a lose caliper that aggravates the slip/stick friction etc.. As I mention that is often the cause of horrible squealing on light brake pressure at low speeds.

Don't know why your brakes squeal only when backing up. May be a different, alignment, brake caliper etc issue. But so easy to do why not try?

SIDEBAR
If you are referring to Pic of Permatex Ceramic Brake Lub (only one I see in the thread in my post #24) it is a Band-Aid only reducing vibration to make it go away for a while. It is ALL a dealer can be reimbursed for by GM. Dealer will NOT do "Burnishing" as is driving safety issue and finding a place not easy. As noted in Pic's I place on the steel sides AND backs of new pads I install.
Burnishing or Bedding corrects the cause and is better than pulling the pads and installing Lub that is only a Band-Aid!