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my mac cat 839 chainsaw oil pump is not delivering oil?

i've stripped down the saw, removed and cleaned oil pump, blown the pipes clear and rebuilt but still no oil. i did notice that the oil pump drive gear spins freely on the shaft and i don't think it should. I also couldn't figure out how the pump actually works!

Any advice on MACCAT 839 or similar would be gratefully received.
mick S, November 2012