1995 Johnson/Evinrude Outboards 40 thru 55 2-Cylinder Service Manual, Page 304Get this manual
THERMAL EXPANSION RELIEF
1Under certain conditions, external temperature variations can cause the unit's hydraulic fluid to expand2The extreme pressure caused by thermal expansion flows from the cylinder and bypasses the UP check valve and the manual release valve venting to the reservoir through the EXPANSION relief valve 3When internal pressure reaches approximately 8000 PSI (55,000 kPa), the EXPANSION relief valve opens and allows fluid to flow into the reservoir