William,
I didn't use a a MPB conversion but recent did a conversion using a master/booster from Old Irish Dave (ebay) (MBM parts) and Hoppers Stoppers calipers and disks (PRB & DBA parts).
What I noticed was that although I did a lot of bleeding I ended up with a pedal like yours. The answer was in the adjustment of the actuation rod to the booster/master. An adjustment of the clevis of only a few turns changes the pedal play by about 1/2". You only need about 1/4" to 1/2" play before you feel resistance of the master then the resistance once the pads make contact with the disks. You need play so that the valve in the booster is open when the brakes are not in use and so that the brakes aren't always on.
Your chasing yourself a bit as you need the brakes bleed to adjust your free play. You can't properly bleed your brakes until you adjust your freeplay. I cycled between the two (bleed, adjust bleed, adjust) a few times to get it right.
Cheers
Warren
Grizzly (Aussie Mainline)