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.