mercruiser tilt/trim problem

[outboarddan]

hi i have a 1976 mercruier 165 i/o and i took outdrive off and trim cylinders to repaint put it back on and checked trim hose connections with another boat and mine are the same, filled hydrulic fluid up and when i go to trim outdrive up it goes up fast about 20 seconds, but when i trim it down it takes about 3-5 minutes to trim down, i have no clue what is wrong there is a vent screw on the resevior and i have left that open, when trimming up and down but has not seemed to make a difference, and when i filled up fluid i did so when it was in the down position, i need your help as this is startibg to **** me off, i have no idea what is wrong.

thanks dan