Air/Hydraulic leaking problem??

Back again this time with a new problem. After having removed the DC spindle motor and disconnecting the hydraulic line to the spindle tool changer cylinder, I keep having to bleed off air after about three uses when changing the tool. I've checked all the connection for tightness and haven't found any leaks, but still the problem persists. I've been bleeding the cylinder at the top of the spindle which has a small screw for this purpose (I think) I keep getting air out of it each time until all I get is hydraulic fluid. What am I missing guys?

Ben
forgot to add that sometimes the hydraulic fluid is pushed out of the clear glass container found down in the cabinet. I did managed to empty this container once when bleeding, could I have an air bubble in the main pump? when I'm bleeding I hold the locking button in until I get air the hold until I tighten the screw back up to prevent it sucking air back into the cylinder.

Or you may have bad seals in one of the units or both.

George