1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 245Get this manual
"UP" Pressure Check IMPORTANT: Insure battery is fully charged before performing tests1Tilt outboard to full "Up" position and engage tilt lock lever2Slowly remove "Fill" plug to bleed pressure from reservoir3Turn Manual Release Valve to turns (counterclockwise) to bleed any remaining pressure4Remove allen plug