Power brakes acting silly!

Another factor I failed to mention is the condition of the brakes. If the rotors are turned thin , pads are worn thin, pistons are out of caliper bores a good distance, you'll have a lot of area to build pressure in. The booster will let you build up to 900 psi in the fluid circuit so sitting still you can make a heck of a lot more braking that is required to stop the car. Amount of area to build pressure in alters the pedals travel though. Add very large bore rear wheel cylinders, all sorts of factors come into play.
Before I bought another booster ( not cheap ), I would dead head the fluid lines at the master cylinder and see what difference it makes.