We had that problem.
Replace the leaky rims with "new" ones from a junkyard.
To get ones with a good seal, pick up rims with tires on them,
where the tire is still holding air pressure.
If the tread is shot, the yard should charge the price for a rim without tire.

I researched 240 tire sizes pretty well.
Most 240s rolling around are on 185/70/14 and some 185/75/14, neither is the right size. To match the original specs for load capacity and for outer diameter (to keep your speedometer accurate), you need 195/70/14 for sedan and 195/75/14 or 205/70/14 for wagons. If you want to match the 185 R 14 spec seen on many 240's, (why??) then you need a 78 or 80 profile value (middle # in the 3-# set), which is difficult or impossible to find.