Check that the tire bead and the rim are free of detritus and smooth.
Tire shops always use water and a soapy substance to help in mounting the tires and then inflate them to let the beads to fit the rims.
Possible that you might have burrs or bumps on the rim edges. Check both inside and outside edges of the rim.
Or the rim is leaking from another source.
Or get the special tire valve that has a rubber fitted washer on the outside that is coupled to a nut that can be tightened, like the W198 300SL's have.
Walter
1967 220SL-diesel-new rims from W123 on withh the pins for the center hubcap.