Make shure the vaccum hose is made for the job or it will suck shut.
Spongy pedal can be caused by several things.
Air in system. might need to bleed at master, or replace master.
loose wheel bearing, or wrong bearing with too big an ID. (I've seen it, after a hard turn the pedal drops)
Sticking caliper, not sliding, if single piston.
Bent pads, or twisted mounting. have someone press on the pedal while you look at the calipers, they should move a little to the side but not twist or rock.
Low vaccum would cause a hard pedal, not low. So I think your problem is probably in the hydraulic side of it.
Check my web page for suspension info:
67 RS 327
69 Camaro Vintage Racer
65 Lola T-70 Can Am Vintage Racer