Caliper Piston Knock Back Springs. Used in low brake pedal situations due to piston knock back caused by vibration, mount or spindle flex. Part # 300-17297 (.72” diameter) is used with all sizes of Wilwood Thermlock pistons and Wilwood’s conventional stainless steel pistons from 1.00” to 1.50” diameter. Spring use in stainless pistons require an updated relief cut not present in earlier versions of pistons. Please check your application before ordering.