1993 Johnson Evinrude "ET" 40 thru 55 Service Manual, P/N 508283, Page 332Get this manual
IMPACT RELIEF 1When the engine hits an underwater obstruction, extreme pressure develops above the piston
2The extreme pressure must be vented and the cylinder rod must extend to allow the engine's gearcase to clear the obstruction3Under extreme pressure, five IMPACT relief valves and one UP relief valve will open to allow fluid to pass through the piston as the cylinder rod extends4To return the engine to its original operating position after impact, the hydraulic unit must be operated in the DOWN direction5To properly absorb the extreme pressure caused by impact, the following components must function correctly: Impact relief valves Up relief valve Filter valve and seat Down check valve