Well, if you are convinced that it is the booster, change it. The only other thing I can think of is in the past, master cylinders for drum brakes employed a residual check valve that maintained a slight pressure in the lines to drum brakes. This hastened the response to the brake pedal. Disc brakes do not have this valve because it would produce drag. The EIS brake service manual I looked at said some systems employ piston cup expanders in the wheel cylinders which provide the same benefits.
