Not happy with new booster.

It looks like your Jegs booster might use a GM-type booster w/ threaded rod, like the $170 ones some have bought on ebay. I suspect your problem is due to simply the length of the rod between the booster and MC. Is there an adjustable nut there, like many boosters have? If not, could you add a spacer? I think it needs a slight slop, so that the booster valve opens and the booster activates before you start trying to move the piston in the MC. It sounds like what is happening now is that you are initially moving the MC piston w/o boost and then the booster kicks in. You probably have a really stiff hydraulic side, with minimal gap before the pads and shoes contact, which is good.