Simple fix first is to take out the ram (from the back) and relube/replace the seals. If that doesn't work take out the poppet and relube it, also check the (white) cup seal that the poppet sits on and reverse/replace that. Those three things normallly are the problem. Although, if you have taken the sleeve out that can cause a similar problem.
Suggest you try the first three in that order and hopefully sort it out. If not give me a pm and I'll try and help you out.