Yes you do need to jack up that rear wheel off the ground and put the tire valve so it is at 12 o"clock position. If you don"t do this, you will probably get a CaCl shower when you disturb the existing valve or valve core. If you have ever had CaCl on your hands, you would understand why you don"t want to get sprayed with that solution. Not pleasant!
Over the last year or so, I had noticed that one of the back tires on my Ford 641D looked like it was a bit lower on air than the other. I drove it to the garage and was going to add some air from the compressor, but when I took off the valve cap, a little bit of solution leaked out. When I tried to add air, none would go in. I thought I might need to replace the valve core, but I could not see anything for my tire valve tool to hook to and it would not remove the core.
So I went to the industrial tire store in town to see what they recommended. The clerk asked me which valve stem I had and showed me a couple of different inserts. He ended up GIVING me a brand new insert, saying he would charge me twice the next time...I WILL be doing my heavy tire business there in the future!
A couple of days later, I noticed that the problem tire looked much lower. I had purposely parked it with the valve stem at the highest point, and I did not see evidence of fluid leakage. I drove the tractor to the garage again and jacked it up, started the compressor and decided to try my luck at removing the old insert. When I first tried turning the old insert, it looked to me like the rest of the stem was turning too, so I got another pair of pliers and took hold of the stem closer to the rim. The insert finally started to unscrew, but I had to use a fair amount of force. But once broken loose, it unscrewed fairly easily, and did not break off, as I had feared it might. I noticed that the O ring that was supposed to be on the old insert was gone, maybe inside the tire or dissolved. The plunger in the old insert was stuck, which was probably why the tire would not take any air. I would have liked to have run a tap into the old threads in the stem, but I did not have one the right size. So I carefully screwed the new stem insert into the old, cruddy looking valve stem, first lubricating the O ring with saliva. The insert went in fine and I tightened it up with the pliers. Then I aired up the tire, which took quite a lot of volume, and let the jack down.
Since I was not sure if the new insert would hold air (or solution), I parked the tractor with the valve stem in the highest position. A couple of days later I checked the tire and it did not seem to have lost any air or solution. Apparently my repair was a success! The tire has stayed fully up for more than a week.
I do not think that insert had been disturbed in more than 30 to 35 years. It had been new when my Dad bought new rear tires on the 641D. I might have added air a few times over all these years, but I am pretty sure that the insert had never been removed.
I was very pleased that I could fix the tire so cheaply. I figured that if the valve stem broke while I was trying to remove the insert, I would end up having to have our local farm tire service come and replace the tube. Probably the better part of $200 before I was done. I know how to change a tire tube, but I am not as young as I once was, and they have the equipment to deal with the CaCl solution. I would have spent the money.
You might get lucky and be able to remove/replace the valve core and have that solve the problem. I was not aware that there was a difference between valve cores for air and for liquid. That is something to know and remember. Or you may find that you need to try changing the insert. I would advise being very careful and watching to see if the rest of the valve stem remains stationary when you try to turn the insert, or if the lower part of the stem moves like mine did. Even though it damages the threads on the lower part of the stem to grasp it with pliers, that is what I figured I had to do rather than tearing the stem loose from the tube. Once the insert was broken loose, it came off pretty easily.
If that is what you have to do, you might want to remove the insert before you go to the tire store, so you can show them exactly what you need.
Good luck, I hope your situation turns out as good as mine did!
:roll: 
