I'm repairing a leaky main boom cyl., and am having trouble bleeding the unit to get the operations back to normal. The cyl. is back in the machine and I can raire the boom ok, but it will not stay up and when I go to lower the boom if I open the valve in the cab all the way it stops the boom from lowering. I think there must be air in the holding valve or something but don't know how to bleed it. Please advise. Thanks