mountain bike disc brakes (need fettling) (Levenshulme M19)

I've got a pair of Shimano SLX disc brakes - levers and calipers, no rotors. They're great when they work but I've had an issue where every few months the pads and rotors get contaminated and need replacing. Maybe there's a leak somewhere although its not obvious to me where. Perhaps someone who's a better mechanic than me could give these a new lease of life - they're fabulous when they work.