in the center of the diaphragm in the booster there is a spool valve there is vacuum on the back side of the diaphragm when you apply the brake pedal it moves that metered spool valve and allows vacuum to move to the apply side and pushes the apply rod to the m.c. now this spool valve is crimped into the diaphragm after old age or non activity for long periods of time the valve will separate from the diaphragm and allow vacuum to leak around the outside and apply too much brake pressure to be applied I have seen this happen before I hope this helps its easier to explain in person than when sit down and try and type it out especially with my limited typing skills bottom line you need a booster ........ted