The front tires on my Sears/Husquvarna kept loosing air, although not as bad as others I've seen. Like my father's pressure washer and a two wheel dolly I have.
Have you tried replacing the valve stem (inserts) and using Green Slime? I put some of that in both the fronts Husquvarna and have yet to loose any air pressure.The tread on the fronts are about bald so it's just a matter of time before they go POP and I'll need to replace them.
I've had this same problem with some of my cars. Although in this case, the tires loose air pressure around the beads. 30+ year old Magnesium and alloys seem to be the worst. I guess from corrosion around the lip of the rim. Never had any issues with steel rims doing this.
I didn't want to use Green Slime in case it threw the balance off so I've removed the valve stem, fully deflated the tire then taken some dishwashing liquid and used one of those acid burshes and applied between the tire and rim then re-inflated it. This trick seems to work well. If the detergent is dried out a bit, seems to work better than if it's new.
