A Body Disc Brake Swap 101

I finished doing this swap on my 73 Dart Sport. The whole thing went real easy and looked great. I picked up all the parts from a salvage yard in CO (thanks to Monty). I decided to switch from manual to power brakes so my daughter can drive it but I now have about an inch and a half pedal travel before the brakes start to catch and it feels weak at best. I picked up a booster and MC from a 72 Valiant and it all seemed to bolt up ok and when I test drove it and felt how mushy the brakes were I put a rebuilt MC on and a adjustable prop block from SSB. Any ideas why the brakes are weak? I can't let me kid drive it until I feel good about it.