301.8 cat slew brake

Hi all got my little 301.8 excavator in the workshop when I slew I hear some clunking noise if I go nice and slow no problem, let go of the lever and I can hear the slew brake jumping missing the spot and clunking until it locks in. I have no idea how it works my back round is diesel mechanic so a little bit. Has anyone got a break down diagram of the slew motor and brake.
Cheers
Chris