thanks!
Thank you all very much for your comments.
I suspected the master cylinder as well, but I was hoping that wasn't it. The air bubble theory also sounds plausible, but I've bled the brakes multiple times. The odd thing is, the problem appears to have resolved itself. The car now brakes like a champ. Beats the heck out of me. The only thing I can figure is that maybe the rears hadn't adjusted (we cranked the parking brake a few times the other day) and now they've clamped down and bedded in properly so the variation is gone.
There is one other problem though: There's a squeaking problem coming from one of the rear hubs. It's periodic with the wheel turning and it's only up to about 20 mph. It also stops when I apply the brakes. I think it might be that one of the pads is off slightly, but I'm not sure. Any ideas?
Thanks again!
Adarsh