1999 "EE" Evinrude 70HP 4-Stroke Service Manual, P/N 787023, Page 180Get this manual
3 Tilt System Pressure Relief
1If engine speed exceeds approximately 1500 RPM when operating in shallow water drive mode, oil pressure will increase underneath the tilt piston The thermal relief valve incorporated in the manual release valve will then open2The oil below the tilt piston will then flow to the reservoir through the thermal relief valve
3As the power of the engine continues to exert downward force on the tilt piston, this will open DOWN check valvethereby allowing oil from the reservoir to flow into the chamber above the tilt piston
4In this way, high internal pressure is relieved and the engine will slowly tilt downward until it reaches the highest position in the trim range Thermal Expansion Relief 1High ambient temperature will, through thermal expansion, induce build-up of oil pressure inside the unit