I would start by making sure there is enough hydraulic fluid in the tank. To do this lift the bed and you should be able to see the tank that holds the hydraulic fluid,it is located between the frames. There should be a fill opening on top with a cap on it.
I bought my new toy,77 GMC 6500 Tilt-Cap dump, about a month ago and am still checking it over learning more about it.
Danny -77 GMC 6500 Tilt-Cab
