2000-2005 Yamaha F40B Outboard Service Manual, Page 398Get this manual
POWER TRIM AND TILT UNIT (8) Remove the reservoir cap 6 and check that fluid is up to the brim as shownAdd fluid if the level is below the brim(9) Install the power trim and tilt reservoir cap(10) Repeat the above steps two or three times until the power trim and tilt fluid is at the correct level