D 15 3 point bleed off

I have a D15 seriesII around 1966 I'm not  exactly sure about that but my books and tractor are about 300 miles away put awawy for the winter. I have a problem with the hydraulics bleeding off the three point hitch and I can't hold any implement up for a long period of time. Plays heck of the bottom of my brushhog deck, besides plowing the field, My question is can I fix this myserlf. I am a qualified hydraulic repairman on industrial tools and equiptment but my barn doesn't have an overhead or much for heavy lifting in it.  I just need to know how much of the tractor I have to take apart to fix this, I have beenvtold there is a ram inside the body but where and how deep do I have to go and is it possible to do this without specialty tools or should I just suck it up and take it to a shop, any help would be appreciated.

I have checked fluid levels for the past two years and it has gotten to the point it has to be dealt with  lost no fluid at all so I figure the cylinder is leaking inside or bleeding down internally and I need to know how get to it to fix it. When I bought this in 1985 I rebuilt the gas engine but never got into the body ,everything has worked great till now

First, you need to determine if the hydraulic compartment oil level is going down and the tramsission going up.  If this is happening, you need a new hose and lift arm ram seals inside the rear end. If both oil levels are staying where they belong, then the problem is the hold valve in the pump itself.

The pump is accessible under the tractor, just ahead of the belt pulley location. Remove the metal plate, 14 or 16 bolts hold it on. Buy a book, and follow it. That will get it out of the torque tube. There are some adjustments that you might want to check first, They are in the book, listed as things to do after you replace the pump. You might find your problem there. I took My pump out to check the cam and followers. Found nothing wrong, reinstalled it, set it up as per the book and everything works now. Bob

Thanks for the info on my problem. I do have all the manuals for my 15 so I'll take your info and make some excuses that I have to go see if I can fix her. Chance to get out of town. Thanks again