1992 Johnson Evinrude "EN" 40 thru 55 Service Manual, P/N 508143, Page 325Get 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 is vented directly to the reservoir by the EXPANSION relief valve built into the manual release valve3When internal pressure reaches approximately 8000 PSI (55,000 kPa), the EXPANSION relief valve opens and allows fluid to flow into the reservoir