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The fluid in the gear boxes on the bush hog are separate from the tractors hydraulics. The tractor hydraulic system runs the lift cylinder to raise and lower the bush hog. The only contaminated fluid will be in the lines from the tractor back to the lift cylinders on the bush hog. The gear boxes on the bush hog are pto driven off of the pto shaft. Drain the fluid from the hydraulic lines and bleed the fluid out of the cylinders and you should be good to go.