A SUPPLY TRIP CONSISTING OF 2 SUPPLY TRAILERS AND BARNEY CAR WAS BEING DROPPED DOWN TO SLOPE BOTTOM. DUE TO MUCK AND WATER ACCUMULATIONS SUPPLY PERSON CONTACTED HOIST OPERATOR TO HOLD TRIP AT 3700 MARKER. THE BRAKES ON THE SLOPE HOIST DID NOT HOLD THE TRIP AND IT PROCEEDED DOWN TO 4200.