The air motor on the 285B needs to be oiled, the hammer section needs to be greased. The symptom described could be in the hammer mechanism, 285B's have 2 different side plates (item# 41) that sit atop the hammer mechanism, tools made from 2005 to 2007 use the old style side plate (item# 41) that has a small lip that need to sit looking toward the hammer case bushing (item# 44) when installed with the small lip facing in to the hammer will cause it to lock, tools made from 2008 to the present have a side plate that is flat on both sides and can be installed in either position. also check the hammer and anvil for cracks and pitting.
Follow this link to view the parts list.
http://manualarchive.ingersollrandproducts.com/ir_pdfs/Tools/Vehicle%20Service%20Tools/Air%20Impact%20Tools/80162944.pdf