I have had something similar happen before when I replaced the front pads. Don't remember if it was my Blazer or my Bonneville, but the firmness got better the more I pumped it.
I think rjblazer told me awhile ago that when you compress the pistons on the calipers to replace the pads and put them back over the rotors, it takes awhile for the fluid to "replenish" itself taking up the extra slack at the rotors. Though they are new and thicker, you did push alot of fluid back into the reservoir.
Dumb question, but did you take the cap off the reservoir when you clamped the calipers?
1997 Pontiac Bonneville SEi-SCRAPYARD
2000 Chevrolet Blazer LT 4X4 4.3L
2002 Chevrolet *-10 LS Ext Cab 4X4 5.7 V8