these should be self adjusting,but the caliper slides are bad for seizing on these so this may be giving you an imbalance on your brakes
wickham
September 2010
rear disc brakes should be self adjusting, are calliper slides siezed or are pads contaminated with oil from axle leak.maybe piston in calliper siezed, could be air in system? load sensing valve [if 2 pipes feeding it] could be faulty.
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