Tractor owners, I do mean tractor too, not lawn mowers.

Front loader and pwr str. powered by the same pump, rear brakes are mech. drum.

 

I think you have a weak/broken spring in the 3 point hitch control valve the more I read and think about it.

It cannot be a pump issue because it is performing work as it is supposed to, it is just diverting oil to the relief valve, or open center setting giving you no power to the front end loader. When the 3 point hitch is not being used the front end loader works as it is supposed to. This tells me your pump is not only shared, but strong.

A weak or broken spring in the 3 point hitch valve body however would do as you describe, for as soon as you start to lift something, there is no spring pressure to hold back the pressure and it immediately dumps oil to the relief valve/open center position. This is an old tractor so if something was plumbed wrong it should have cropped up long before this point.

On my bulldozer I can adjust pressures for worn springs on all my valve bodies, but that is a horse of a different color I know. I might check into that before removing the 3 point hitch valve block as it would be easier to adjust by checking adjustments with it on the tractor. If you cannot adjust the valves either by an adjustable orifice, or by shimming the valve, I would take the valve block on and put it on the bench and look deeper into it. If you are comfortable digging into it, do so as they look more intimidating then they really are. Just keep in mind these key things:

Cleanliness is next to Godliness with hydraulics. Keep everything spotless
Take lots of pictures before disassembly, this helps in reassembling everything
Have respect for the valves bodies; a lot of pressure goes through very small holes and parts. Take your time in rebuilding stuff'

Here in Maine at least, hydraulic shops are everywhere. They can rebuild your valve body and determine the problem if you are uncomfortable digging into the valve body.

This past winter on my skidder I kept having the winch pull in when it was on "hold" and I was driving out of the woods. Finally it would not release either unless the skidder was revved up, yet when put on hold it would constantly winch in. In the end it was a tiny spring that had broke inside the valve body. The point is, a tiny $100 spring can make even a 150 hp skidder rendered useless.

 

Thank you sir, That's some great technical help. I'm gonna make some pressure port checks and see what the verdict is and go from there. Your most likely correct on valve body issues. I'll surly post what I find out. Thanks