My father-in-law picked an old D31A last fall to make some logging roads. After using it for a few weeks the blade angle can no longer be adjusted. He has tried bleeding the obvious parts and with it powered off you can bleed it out and the blade goes down. But once you start going the blade goes back to the tilt. Any general ideas of what this could be and a fix for it. He thinks the guy he bought it from said he had the problem previously or something at least something like it, but he doesn't have a way to contact him. Thanks for any advice.
If I had to guess...and I am guessing here, I say you have a blade control valve problem, and fluid is bypassing in the valve bank and pressuring the tilt control even though the handle isn't directing it to do that.