According to the Makita Hammer Drill HM0870CQ instruction manual (available online in pdf), when the brushes are changed, the grease should also be changed. This seems to be a common feature with demolition hammers. Also, if the tool was stored in a cold area, it may have to be run for five or more minutes to warm up the grease enough for the tool to hammer. If the tool ran , there was probably nothing wrong with the brushes; the grease was either cold or depleted. Do an internet search of <Makita Hammer Drill HM0870CQ instruction manual> . This will probably tell you all you need to know.