I just replaced my booster on my '64, and the return spring is internal to the unit. The spring on the floor more proably fell out of the swing-away steering wheel doors (I think).
You might check the little rubber boot on the firewall that covers the push rodd from the brake pedal to the booster unit. It could be binding in there somehow. Mine was rotting and folded up funny (but not binding) when I took out the booster unit.
Another thought, possibly the binding is coming from the master cylinder. It might be worth your time to remove the master cylinder (which is attached to the front of the booster under the hood) and look at the rod coming from the booster into the unit. Perhaps it got bound up with old fluid or even dirt and debris. If so, that might hinder the return of the brake pedal.
One last thought, maybe the hinge on the brake pedal just needs a shot of oil. Does it squeak or scrape when you depress the pedal?
Good luck!
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