No offense intended:
You do know that the rear pistons typically don't just push in like the fronts, right? Because they are also your park brakes, they turn in.
Now, mind you, I haven't actually change the rear pads on my bonne yet, but on dozens of other GM cars that I have done with rear calipers, you should use the tool that allows you to twist them in. Is the face of the piston flat, with two groove in it? If so, this is the twist in type. (I wish I could remember from when I pulled mine apart to paint the calipers)
(If you did know this, but still having problems, again no offense intended. )
Proud owner of 'Quicksilver'
Currently Own - 2000 Bonneville SSEi, 70K, Silver (the fastest color)
Factory Options: all, except seat heaters (See Below)
Mods: Custom Seat heaters, GXP Strut Tower bar, Solid Engine mount (Alum Roundstock/ tire sidewall hybrid), Intense FWI, Intense 3.4" Pulley, Over-size Idler Pulley (for stock superchareger belt size), Mr. Alternator power source (Powder Coated Red), Belden Wires, Red Wire Looming, Autotap OBDII Gauge.
Appearance Mods: BRIGHT Red Brake Calipers all around, Cleared Corners, GXP Tails, Red Wire Looming, 17" Chrome 5 Spoke Rims
Next Mods: Intense PCM and Stage 1x Cam