A 65-ft.-high drill rig, mounted on a freshwater barge, was allowing its top drive head to drop uncontrollably. The circuit controlling this function shows (see illustration) both a brake (counterbalance) valve and a hydraulically controlled mechanical brake on the hoist function. The systems’ power unit was mounted close to the deck, with its A and B line hoses going up to top drive’s hydraulic motor. The brake and shuttle valve were plumbed close to
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