'Weighting' the rear tires of 4wd Kubota???

You paid how much???

My SO filled the tractor tires with water several years ago.
There’s an attachment that goes on the end of your garden hose (bought at JD for under $20).
Remove the valve from the tire, attach the garden hose to the valve stem and fill the tire’s 3/4’s full of water, the rest is air. Put the valve back in the tire.
Drive!

We’ve never had to refill and no leakage.

Try contacting Tom King, a specialist in remodeling old houses.
This poster is not active any more, but he is very nice:

https://www.chronofhorse.com/forum/member/99547-tom-king

He may know what you have there, or know who where you are is good with old houses, that may help you.