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. BTW, foolishly drove for 6 months cleaving the Z51 Bembo pad dust every few trips to town before installing low dust Carbotech 1521 pads! But about every day used the brakes aggressively as recommended to maintain that uniform pad layer on the rotor. Never had another squeal!

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!

 

The Service Manual has instructions for burnishing the brakes, both for the standard brakes and the heavy duty brakes. Both are indicated for new pads and/or new or refinished rotors. They do not say they will eliminate squeal (even though they sometimes can reduce it). In fact, the instructions for the heavy duty brakes specifically says squeal is normal. Neither procedure is called out specifically to address brake squeal. It also says this about that: ""Squeak/Squeal Noise - A squeak or squeal noise can occur on vehicles with front semi-metalic brake pads during light to medium brake pedal application at low to moderate vehicle speeds. Occasionally this noise may appear from the front or rear brakes after a period of non-use such as overnight, or cold brakes and/or in high humidity conditions."
For pad installation, they both call for the use of anti seize lubricant on the back of the pad where it contacts the pistons. Form the J55 procedure:
Apply approximately 0.09-0.15 g (0.003-0.005 oz) of anti-seize lubricant to each end of the disc brake pads abutments which contact the brake caliper.
From the standard brake pad installation:
Apply approximately 0.09-0.15 g (0.003-0.005 oz) of anti-seize lubricant to each end of the disc brake pads abutments which contact the brake caliper.

Brake Pad and Rotor Burnishing (JL9 & J57)

WARNING: Road test a vehicle under safe conditions and while obeying all traffic laws. Do not attempt any maneuvers that could jeopardize vehicle control. Failure to adhere to these precautions could lead to serious personal injury and vehicle damage.

Burnishing the brake pads and brake rotors is necessary in order to ensure that the braking surfaces are properly prepared after service has been performed on the disc brake system.

This procedure should be performed whenever the disc brake rotors have been refinished or replaced, and/or whenever the disc brake pads have been replaced.

1. Select a smooth road with little or no traffic.
2. Accelerate the vehicle to 48 km/h (30 mph).
   NOTE: Use care to avoid overheating the brakes while performing this step.
   
3. Using moderate to firm pressure, apply the brakes to bring the vehicle to a stop. Do not allow the brakes to lock.
4. Repeat steps 2 and 3 until approximately 20 stops have been completed. Allow sufficient cooling periods between stops in order to properly burnish the brake pads and rotors.

Brake Pad and Rotor Burnishing (J55 & J56)

WARNING: Road test a vehicle under safe conditions and while obeying all traffic laws. Do not attempt any maneuvers that could jeopardize vehicle control. Failure to adhere to these precautions could lead to serious personal injury and vehicle damage.

Burnishing the brake pads and brake rotors is necessary in order to ensure that the braking surfaces are properly prepared after service has been performed on the disc brake system or if pitting occurs on the brake rotor's surface.

This procedure should be performed whenever the disc brake rotors have been refinished or replaced, and/or whenever the disc brake pads have been replaced.
NOTE: Use care to avoid overheating the brake while performing this step.

NOTE: As with all high performance brake systems, some amount of brake squeal is normal.

1. Use the Instrument Cluster's DIC performance menu's friction bubble that calls out longitudinal G forces to determine braking forces. Use the Friction Bubble in the DIC display to determine the correct deceleration rates. Apply the brakes 10 times starting at 100 km/h (60 mph) to 50 km/h (30 mph) while decelerating at 0.4g. This a medium brake application. Drive for at least 0.5 km (0.3 mi) between applying the brakes.
2. If further cleanup of the brake discs is needed, repeat this procedure with 0.7g applications.

 

More aggressive pads can make noise.

Light use of aggressive pads strips the transfer layer - which is even more noise (and rotor wear)

You are using the product in a manner in which it was not intended.

And my pet peeve - grease. It specifically startes "each end of the disc brake pads abutments which contact the brake caliper"

Nothing about slathering it on the backing plate making a giant mess. These is no movement there and furthermore how does grease dampen a vibration?

Just curious.

My very aggressive track pads do not squeal unless I leave them on for an extended period on the street and drive lightly.

 

This...