Slowly running empty tires so ..
The most obvious reason for this is leakage through or along the valve or through the rim rim. (check with suds)
Leakage through the valve can be due to aging or contamination. Years of inflating a tire with air with a high level of humidity (for years forgetting to clean the air barrel of the compressor of condensation), aging of the rubber profile with which the valve hugs the valve hole and corrosion of the rim edge, a phenomenon that is especially common with heavy-duty light metals rims occurs.
A clean inside work in the valve is no problem, our advertisers provide handy tools to put new valves in the rim. Remove the rust from the valve hole before the new valve gets its place and immediately give it a lick of Vaseline.
The leakage along the rim edge can be remedied by sanding the rim edge well.