Rear brake calipers

[Brit Trucker]

Hi everyone, my wife has a 2005 2.2l diesel sport Almera and no other car exists for her. We live in France and if its not standard or off the shelf it does'nt exist. We bought her in 2010 and have had very few problems with her EXCEPT the rear calipers keep seizing up. Taken them off and cleaned, then replaced them along with 3 sets of discs. We live in the mountains and the Almera handles like a dream but as you must understand stopping is pretty important as well as not having glow in the dark discs. Has anyone else had this problem and how did you rectify it. We want to keep her and i've been thinking of upgrading the brakes and soon the shocks. Im new here and my cars a pick up for off roading so im a bit out of my depth here. Any advice other than get rid of her (Almera not my wife) would be greatly appreciated