If you lock it with the small lever on the inside does it lock properly?
If it does then the internal mechanism is probably ok and the barrel may be the problem. It is most likely though that you have either worn internals or even a broken spring inside. It's not that hard to take them apart and clean and check them but you have to drill the rivet out and then pop rivet them back together afterwards. 
I did some repairs on some of mine once and still ended up buying a complete new set from the UK as they still wouldn't stay locked  . I think the dollar was a little better against the pound then though.
. I think the dollar was a little better against the pound then though.
Cheers,
TimJ.
				
			 
			
		 
			
				
			
			
				Snowy - 2010 Range Rover Vogue
Clancy - 1978 Series III SWB Game.
Henry - 1976 S3 Trayback Ute with 186 Holden
Gumnut - 1953 Series I 80"
Poverty - 1958 Series I 88"
Barney - 1979 S3 GS ex ADF with 300tdi
Arnie - 1975 710M Pinzgauer
			
			
		 
	
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