During the daily inspection prior to a series of glider towing operations, the pilot noticed that the brake linings were worn. During the landing roll following the third of these operations, the right brake failed and the aircraft ground looped before coming to rest. The brake had failed following the loss of hydraulic fluid from the seal for the brake caliper piston. This was caused by excessive piston travel, associated with severely worn brake linings. When the assembly was dismantled, it was found that the linings had worn completely off the backing plate, and those on the pressure plate were only 1.1 millimetres in depth. Both brake discs were pitted from the effects of corrosion, which would have caused the excessive wear in the linings.