Caliper Piston Knock Back Springs. Used in low brake pedal situations due to piston knock back caused by vibration, mount or spindle flex. Part # 300-12995 (.72” diameter) is used with all Wilwood Thermlock pistons and Wilwood’s conventional stainless steel pistons from 1.00” to 1.38” diameter. Spring use in stainless pistons require a relief cut not present in earlier versions of pistons. Please check your application before ordering.