'Grabby' brakes

The reaction valve is the brain of the booster. If the pushrod length is right, the brain accepts your leg input and tells the booster how much boost to add to your leg pressure. If the P-rod is too long it will boost too early. If too short; too late. This is a mechanical device, and therefore stupid. It only responds to the push-rod length.Its possible to break it with a very-much-too-long adjustment. If its waaay too short(never tried it) it may not boost at all, but I doubt that.
Inside the booster is a big rubber-type diaphragm, running down the center. On the one side is atmospheric pressure.With the engine running and connected up, the air on the other side gets evacuated. So now we have a pressure differential in there. Which we can use to perform work. In the center of the diaphragm is stuck the reaction valve.When the P-rod acts on it, it re-acts by adding its boost to your leg pressure.