This can be caused by a few things. Most likely it is a sticking control valve at the handle. C
But it is way too complicated to write it all out here.
What repair manual/s do you have for that tractor? You MUST have at least the small (150 page) I&T FO-31 manual and spend some time studying it and THEN ask your questions here.
You will also need a gauge that will read about 1000 lbs.
You will need to pull the lift cover off to check the pressure.
This site sells both the manual and the lift cover casket which you will need.
If you have a credit card you can call them at 800 853 2651 and get them shipped to you.
The gauge can be ordered from Northern tool. You will need to make up a couple of fittings to get it to hook up into your tractor. Napa should have some brake line and fittings that will do the job.
I had the same problem about 3 or 4 years ago and figured it out with the help of this board.
You can too.