Eric, when you say no power on first brake, are you getting a soft pedal, or do you imply the pads have to warm up before grabbing? If you are getting a soft pedal, you may be experiencing some "pad knock back", likely on the rear axle, but also possible on the front if the wheel bearings are set a bit loose. The quick fix in use is to pump the pedal slightly before use and see if that helps. I have found it to be a rigorous endeavor to set the rear axle end play at a minimal amount as GM specifies. (one good reason for full floater conversions) If indeed you are experiencing the pad knock back, have your mechanic check the axle end play to see where you are at.
Are you using the 1inch bore master cylinder?