POWER BRAKE VACUUM

A Booster is not gonna solve spongy.
Spongy can be :
Air
hoses
Plumbed backwards at the Combination-valve
calipers not aligned with disc
pads; bent, twisted, flexing,

Do yourself a big big favor;
Get all the calipers off the brackets, and clamp each piston into the bottom of the caliper bore with a big sturdy C-clamp; Be careful as this can overfill the reservoir as the fluid backs up; you might have to take some out. Yes there are alternatives but I won't recommend them, this late in the game.
This eliminates everything but air and hose flex, so if you got all the air out, after a pump or two, your pedal will now be hard and high and stay high.
> if the pedal will not come up high, then the compensating port is not working
>If the pedal slowly falls under your foot at moderate pressure, with NO visible external leaks, the master is bypassing internally and needs to come apart.
>If the pedal is mildly spongy, check for flex-hose movement; a little swelling is normal; it should be just barely perceptible
>>If after eliminating all of the above, and she is still spongy and your C-clamps are sturdy, then; sorry man you still got air.

Did you bench bleed the M/C?