N.H. 6640 three point hitch won't go down

[cashtech]

I have a 1997 New Holland 6640 cab tractor. I let a friend use it for mowing. He hooked up an offset tiger flail mower and worked well for about 2 days. Then he brought it back to me and let me know that the three point hitch will not go down with the tractor running. If I shut the engine off I can let it down using the control lever. But as soon as I start it the hitch goes up no matter what position the control is in. Also, after a few minutes of running it loses nearly all hydraulic pressure out of the remote couplers. I pulled and disassembled the priority valve and can't find anything wrong.

Any ideas?

 

[whodat526]

take the priority valve and use a brake cylinder hone to clean it real good and them flush with ether. the priority valve is most likely the trouble