What was found
(CAN) HAD A HARD OVER CYCLIC PROBLEM WITH AN ENGINE, THE AIRCRAFT WAS EQUIPPED WITH MFG SERVOS. HAD THIS PROBLEM AT 30 DEGREES C TO -15 DEGREES C (OUTSIDE TEMPERATURE), NO DIFFERENCE. BOLT LATERAL SERVOS WAS DUE FOR OVERHAUL, SEND SERVOS FOR OVERHAUL AND REINSTALLED THESE SERVOS AFTER OVERHAUL, NOTHING CHANGED. REPLACED THE VALVE FILTER BLOCK AND NOTHING CHANGE. REPLACED THE LT INPUT CHECK VALVE AT THE SERVO AND HAD A GAIN OF 60 PERCENT. REPLACED THE RT INPUT CHECK VALVE AT THE SERVO AND HAD A GAIN OF 100 PERCENT, NO MORE FEED BACK. FOUND THE O-RING INSIDE OF THE CHECK VALVE (FLAT).