I have a Yanmar 1500 and just got done changing the transmission/hydraulic fluid and filter and went t o raise and lower the 3 point hitch. It went up but would not go down. I didn't have an attachment on it so at first i thought it wouldn't go down because it didn't have any weight on it. I put my weight on it with the lever in the lower position but it didn't budge. Could I have somehow got it stuck in the up position because I operated it without an attachment on it? Could there be air in the system from when I drained the fluid? Dump valve failed?