HOW did you make room in the calipers for the new, thicker pads?
In general, the worst, most-contaminated, wet, sludgy fluid in the brake system is at the lowest points--the calipers or wheel cylinders.
Did you push the pistons back into the bores, forcing crappy, contaminated brake fluid backwards through the ABS unit?
Or did you pinch the rubber brake hose closed, open the bleeder screw, and blow the contaminated fluid out the bleeder screw as the pistons went back into the bores?
Getting rid of contaminated fluid requires a brake fluid flush; and that's a great idea any time the friction materials are replaced. Clean out the plastic reservoir on the master cylinder, put in fresh fluid, bleed RR brake until fluid runs clean at the RR bleeder screw. It'll take awhile. LR should take a bit less time because the rear brake system should be clean except for the LR split behind the rear brake hose. RF will take awhile, LF about the same. But that only flushes three out of six valves in the ABS unit. To flush the other three, you'll need a scan tool capable of performing the "automated ABS bleed sequence"; the tool cycles the ABS valves and the ABS fluid pump. It gets the air and contaminated fluid out of the ABS unit, but not out of the brake system, you'd have to bleed all four brakes AGAIN to move the air and old fluid down and out the bleeder screws. GM says you need a gallon of fluid to do this properly--half-gallon before the ABS bleed, half-gallon after.
And then you hope there aren't rust particles or other debris trapped in the ABS valves that don't get flushed-out.