Weak, spongey front brake after installing new pads

[ashicus]

Semi-novice mechanic here in need of advice. I recently installed new pads on my friend's 2008 GZ and now the front brake is super spongey. In fact I can almost pull the lever all the way to the grip if I squeeze hard enough. The brake *is* working, but it's feels really weak. Here's the process I used:

• Removed caliper and old pads

• Used a soft cloth (dampened with a bit of kerosene) to clean around the inside of the caliper and the outside of the piston

• Used a small c-clamp and an old pad to push the piston back in. I did *not* open the bleeder valve or remove the cap of the fluid reservoir (my hunch is that this is where I screwed up)

• Applied caliper grease to the backs of the new pads and installed them.

• Reinstalled caliper and torqued everything to spec

I used the same process when replacing the pads on my Bonneville, and didn't have any issues, so I thought I'd be good. Did I mess up by not opening the bleeder?

Any advice greatly appreciated