Caliper Piston Knock Back Springs. Used in low brake pedal situations due to piston knock back caused by vibration, mount or spindle flex. Part # 300-2434 (1.22†diameter) is used with Wilwood’s conventional stainless steel pistons from 1.62†to 1.88†diameter. Spring use in stainless pistons require a relief cut not present in earlier versions of pistons. Please check your application before ordering.