The power assist comes from the PS pump. They use a "nitrogen ball" to store some hydraulic energy so you get an assisted stop or two if the boost fails. There are pre-made kits with all new parts for '70's B & E models (spendy). For PU's, I believe the system needs to be adapted from another model or pieced together.

http://www.hydratechbraking.com/brakeunits.html#TXTOBJ7D920F13152E1B71