2.5DT drive belt tensioner.

i just bought a new tensioner, and it does nothing. i don't know if im putting the oil for the tensioner right or if it is the right amount. its giving me serious pain in the backside. i would be grateful if someone could explain how to bleed it properly so when i put it under tension it doesnt loose pressure. (i fill it and when i put the fan belt in place the thing just lets al the fluid out.)