I currently have a 1969 Cat 12E 17K that we use for farm grading and contract work In Aus. Was doing some fairly heavy work today and suddenly the left blade arm stopped lifting or lowering and would only kick lever when trying to raise or lower the blade. I assume I have a worn dog clutch but any thoughts or tips would be very much appreciated

Cheers   

Well that is a good place to start. Jumping, kicking back or knuckle busting is a sign of worn dogs but there may also be an underlying issue caused by a drag from a bad bearing, in need of grease or low oil in a arm lift worm gear. Somewhere between the control box and the moldboard may be seizing.
Good luck.

  Just an update for everyone who is interested and thanks for your reply Dan. 

Today started by checking dogs but they are in excellent condition considering the age. Next moved on to lift arm assemble and found that the lift arm worm has broken apart. Luckily have been able to find parts quickly and should be up and burning diesel again next week. 

I have driven Cat graders (all models) across Australia in the outback and by far this is the best farm machine I have ever owned. Will push dirt all day long with no complaints.