The GM brackets support the caliper so that the thrust of the caliper is taken between the pads--balanced, so no twisting force. The pins and sleeves allow side-to-side movement only.
OE GM knuckle designs support the caliper / pads so that the braking (frictional) forces are conveyed to the suspension. Here's a picture illustrating the abutment built into a GM knuckle for a GenII F-body, but many others are similar. It's aligned with the rotor. The frictional forces from braking are transferred into the abutment - knuckle - suspension.
Compare this to, e.g., the Little Shop disc conversion (2nd picture below), although others are similar. The caliper they use (at least in this video on YouTube) has the machined flats on it, front and rear, for the abutments of an OE design; that's how the calipers are intended to be supported. But, unlike the OE knuckle on the GM F-body pictured, Little Shop's brackets do not supply support in this area. The frictional forces are counteracted by only pins bolted onto the Little Shop bracket, and that's a couple inches or so offset from the rotor. These offset forces result in a twisting moment applied to the caliper that is counteracted by the pads and rotor. The OE design does not operate in this way.
Picture below illustrates the GM GenII F-body lower portion of the front caliper, and the abutment on the knuckle which manages the braking (frictional) forces. Top of caliper is similarly supported.
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Picture below is the Little Shop kit with supplied caliper. Notice machined flat on caliper where knuckle abutments would support it in an OE application.
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The Little Shop brackets rely on the pins/sleeves to retain the caliper entirely, and caliper thrust is taken way off-center, so the pins bend with brake application. The pins were never intended to accept thrust loading or bending forces.
As illustrated and noted above.
Again, I don't know about the Lugnutz conversion--but I suspect it's the same as the Little Shop.
From what I've seen on their site it's identical.
Discs don't self-adjust if the caliper piston and square-cut seal aren't in good condition; and some calipers need the park brake worked to stay in adjustment.
Adding: The pins need to slide freely as well.
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