Like the old saying goes if it isn't broke don't fix it and in the case of these old tires holds true for valve stems. Since you have weighted tires and the valve is made from brass or metal just means it probably has rusted or the internal o-ring has dry rotted. So it may seep. You can replace the internal valve for a few dollars which should resolve your leaking problem but on the other hand you might remove the valve and find out the threads are rusted away and you can't reinstall the new one. I would leave it alone unless you use this tractor allot. Go over to TSC or your local tractor store and they will have all the vavles etc.