Power brake/ pedal grabs, releases, then grabs

[kensousa]

I've got a '71 with factory PS that have been working just fine until now.
So when I touch the pedal, there's the right amount of resistance and the brakes begin to drag.
Then as I continue to push down, the resistance goes away and the pedal sinks very rapidly.
And since I can't let up my foot that quickly the pressure then pushes the pedal all the way down, almost locking up the wheels and trying to faceplant me into the windshield.
The brakes seem to working just fine
, it's just this varying resistance on the pedal that I can't control.
Should I bite the bullet and buy a new booster, or has anybody experienced this and have a different solution?
Thanks