hydraulic lift

[slingshot]

have a cmc lift on my boat. It will trim up about enough to put my motor brace on but thats about it then it starts to bogg down almost like my battery starts to die[ I know its not my battery]. Don't know if I have lost some fluid or have air in the lines. Either way it looks like a sealed unit except where the 2 hard lines come out of the bottom. I see no place to add fluid. any ideas?